Industrial Environmental Restv.rc'1 : !"'*'
on Laboratory ' '• ' -
Research Triangle Park NC 277' 1
vvEPA
Operation and
Maintenance of
Participate Control
Devices in Kraft Pulp
Mill and Crushed Stone
Industries
-------
RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental Protec-
tion Agency, have been grouped into nine series. These nine broad categories were
established to facilitate further development and application of environmental tech-
nology. Elimination of traditional grouping was consciously planned to foster technology
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1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
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6. Scientific and Technical Assessment Reports (STAR)
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9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECHNOLOGY
series. This series describes research performed to develop and demonstrate instrumen-
tation, equipment, and methodology to repair or prevent environmental degradation from
point and non-point sources of pollution. This work provides the new or improved tech-
nology required for the control and treatment of pollution sources to meet environmental
quality standards.
REVIEW NOTICE
This report has been reviewed by the U.S. Environmental
Protection Agency, and approved for publication. Approval
does not signify that the contents necessarily reflect the
views and policy of the Agency, nor does mention of trade
names or commercial products constitute endorsement or
recommendation for use.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161
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EPA-600/2-78-210
October 1978
Operation and Maintenance of
Participate Control Devices in Kraft
Pulp Mill and Crushed Stone
Industries
by
M. F. Szabo and R. W. Gerstle
PEDCo. Environmental Specialists, Inc.
11499 Chester Road
Cincinnati, Ohio 45246
Contract No. 68-02-2105
ROAP 21ADL-037
Program Element No. 1ABO12
EPA Project Officer. Dennis C. Drehmel
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
-------
ABSTRACT
Control of fine particulate emissions from selected kraft
pulp mill and stone crushing facilities is addressed. The
principal devices considered are electrostatic precipitators,
wet scrubbers, and fabric filters. Guidelines are provided for
industrial personnel responsible for selection of an appropriate
control device. Information on the operation and expected per-
formance of conventional air pollution control devices is based
on current design practice, theoretical design models, reported
performance, cost predictions, and published information.
-------
TABLE OF CONTENTS
Page
Abstract ii
Figures vi
Tables x
Acknowledgment xiv
1.0 Introduction 1-1
1.1 Purpose of Report 1-1
1.2 Significance of Particulate Emissions 1-1
1.3 Scope of the Report 1-2
2.0 Control Systems: Parameters and Correlations 2-1
2.1 Emission Sources, Characteristics, and
Control Systems 2-1
2.2 Evaluation of Various Control Alternatives 2-43
2.3 Electrostatic Precipitators 2-43
2.4 Mechanical Collectors 2-81
2.5 Wet Scrubbers 2-84
2.6 Fabric Filters 2-100
3.0 Operation and Maintenance of Particulate
Control Devices 3-1
3.1 Operation and Maintenance of Electrostatic
Precipitators 3-1
111
-------
TABLE OF CONTENTS (continued)
Page
3.2 Maintenance and Operation of Mechanical
Collectors Servicing Bark/Fossil-Fuel Boilers 3-16
3.3 Operation and Maintenance of Wet Scrubbers 3-20
3.4 Operation and Maintenance of Fabric Filters 3-35
4.0 Fractional Efficiency Relationships 4-1
4.1 Introduction 4-1
4.2 Procedures for Determining Fractional
Efficiency Performance 4-2
4.3 Efficiency Relationships for Fabric Filters 4-20
5.0 Summary and Conclusions 5-1
5.1 Design Parameters 5-1
5.2 Operation and Maintenance 5-9
5.3 Fractional Efficiency Relationships 5-11
5.4 Costs 5-14
Appendix A-l Installation Lists for Particulate Control
Devices on Kraft Pulp Mill Applications A-l
Appendix A-2 Capital and Annual Costs of Precipitators
for Pulp Mill Applications A-10
Appendix A-3 Checklist for Obtaining Design and
Operating Data on Particulate Scrubbers
(Design and Operating Parameters) A-17
Appendix A-4 Capital and Annual Costs of Venturi
Scrubbers on Kraft Pulp Mill and Crushed
Stone Industry Processes A-20
IV
-------
TABLE OF CONTENTS (continued)
Page
Appendix B-l Electrostatic Precipitator Subsystem and
Component Function and Operation B-l
Appendix B-2 Preoperating Checklist for Precipitators B-15
Appendix B-3 Electrostatic Precipitator Inspection,
.Maintenance, and Troubleshooting Procedures B-20
-------
LIST OF FIGURES
No. Page
2-1 Kraft Pulping Process 2-2
2-2 Principal Design of Recovery Boiler Furnace 2-5
2-3 Direct-contact (Conventional) Recovery Furnace
System with Black Liquor Oxidation 2-6
2-4 Indirect-contact Recovery Furnace System 2-7
2-5 Material Flows Through a Typical Crushed Stone Plant 2-26
2-6 Particulate Size Distribution from Rock Processing
Operations 2-31
2-7 Combination Wet Suppression Dry Collection Systems 2-35
2-8 Wet Dust-suppression Systems 2-37
2-9 Hood Configuration Used to Control a Cone Crusher 2-41
2-10 Sectionalization of a Precipitator 2-55
2-11 High Tension Splits 2-58
2-12 Plot of K Versus -ln(l-r,) for Recovery Furnace
Applications 2-68
2-13 Selected Precipitator Correlations for
Conventional Kraft Pulp Mill Recovery Furnaces 2-72
2-14 Selected Precipitator Correlations for Low-odor
Kraft Pulp Mill Recovery Furnaces 2-77
2-15 Selected Precipitator Design Correlations for
Bark Combination Fossil Fuel Fired Boilers 2-81
2-16 Typical Fractional Efficiency Curve 2-85
2-17 Venturi Flooded Disc Scrubber System 2-101
VI
-------
LIST OF FIGURES (continued).
No._ Page
2-18 Exhaust Gas Volumes at Various Plant Capacities 2-119
2-19 Cost-effectiveness of Fabric Filter Systems for
Model Processes 2-127
3-1 Wet Bottom Electrostatic Precipitator with Heat
Jacket 3-5
3-2 Research-Cottrell Flooded Disc Scrubber 3-22
3-3 Reverse Air or Shaker Type 3-37
3-4 Pulse Jet Type 3-38
3-5 Diagram Showing Normal Operation and Shake
Cleaning of a Fabric Filter 3-40
- 3-6 Schematic for Reverse Flow Cleaning During
Continuous Filter Operation 3-42
3-7 Poppet Valve 3-51
3-8 Typical Trough Hopper and Screw Conveyor
Arrangement 3-54
3-9 Bag-cell Plate Attachments 3-55
3-10 Typical Shaker Arrangement 3-59
4-1 Predicted Precipitator Penetration for Conven-
tional and Low-odor Recovery Furnaces. 4-4
4-2 Predicted Precipitator Penetration for Conven-
tional and Low-odor Recovery Furnaces. 4-6
4-3 Predicted Precipitator Penetration for Conven-
tional and Low-odor Recovery Furnaces 4-7
4-4 Measured and Theoretically Calculated Fractional
Efficiency of an ESP on a Kraft Pulp Mill
Recovery Boiler 4-9
vii
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LIST OF FIGURES (continued)
No. Page
4-5 Penetration as a Function of Particle Size for
an ESP on a Kraft Pulp Mill Recovery Boiler 4-10
4-6 Fractional Collection Efficiency of Precipitator
Collecting Particulate from Pulp Mill Recovery
Boiler 4-11
4-7 Predicted Precipitator Penetration for Bark/Fossil
Fuel-fired Boilers 4-14
4-8 Predicted Precipitator Penetration for Bark/Fossil
Fuel-fired Boilers 4-15
4-9 Predicted Precipitator Penetration for Bark/Fossil
Fuel-fired Boilers 4-16
4-10 Predicted Penetration for Venturi Scrubbers on
Sludge Lime Kilns 4-19
4-11 Predicted Penetration for Venturi Scrubbers on
Jaw Crushers 4-21
4-12 Predicted Penetration for Venturi Scrubbers on
Conveyors and Screens 4-22
A.2-1 Capital Investment for Precipitators on
Conventional (High-odor) Recovery Furnaces A-ll
A.2-2 Annual Costs for Precipitators on Conventional
(High-odor) Recqvery Furnaces A-12
A. 2-3 Capital Investment for Precipitators on
Low-odor Recovery Furnaces A-13
A.2-4 Annual Costs for Precipitators on Low-odor
Recovery Furnaces A-14
A.2-5 Capital Investment for Precipitators on Bark
Combination Bark Fossil Fuel-fired Boilers A-15
VI11
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LIST OF FIGURES (continued)
No. Page
A.2-6 Annual Costs for Precipitators on Bark
Combination Bark Fossil Fuel-fired Boilers A-16
A.4-1 Effects of Collection Efficiency and Gas Rate
on the Capital Investment of Venturi Scrubber
Systems for Lime Kilns A-21
A.4-2 Effects of Gas Rate, Electricity Usage, Fixed
Charges, and Others on the Annual Costs of
Venturi Scrubber Systems for Lime Kiln A-22
A.4-3 Effects of Collection Efficiency and Gas Rate
on the Capital Investment of Venturi Scrubber
Systems for Stone Crushers A-23
A.4-4 Effects of Gas Rate, Electricity Usage, Fixed
Charge, and Others on the Annual Costs of
Venturi Scrubber Systems for Stone Crushers A-24
A.4-5 Effects of Collection Efficiency and Gas Rate
on the Capital Investment of Venturi Scrubber
Systems for Stone Crushing Conveyors A-25
A. 4-6 Effects of Gas Rate, Electricity Usage, Fixed
Charges, and Others on the Annual Cost of
Venturi Scrubber Systems for Stone Crushing
Conveyors A-26
B.l-1 Insulator, Vibrator-rapper Assembly, and
Precipitator High-voltage Frame B-4
B.I-2 Precipitator Insulator and Vibrator-rapper
Assembly B-5
B.I-3 SCR Mainline Control B-6
B.l-4 Discharge Electrode Unshrouded B-13
B.l-5 Discharge Electrode Shrouded B-13
B.l-6 Precipitator Collecting Electrodes B-13
ix
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LIST OF TABLES
No. Page
2-1 Typical Gas Characteristics In Kraft Pulp Mill
Processes 2-11
2-2 Characteristics of Particulate Emissions from
Selected U.S. Power Boilers 2-12
2-3 Typical Particle Size Distribution of Fly Ash
from Bark/Fossil-fuel Boilers 2-17
2-4 Reported Uncontrolled Emissions Versus Emission
Factors 2-29
2-5 Factors Bearing on Control Device Selection 2-45
2-6 Parameters Affecting Precipitator Design 2-46
2-7 Design Power Density 2-50
2-8 Design Parameters and Design Categories for
Electrostatic Precipitators 2-51
2-9 Typical Operating Conditions for Precipitators
on Conventional Recovery Furnaces 2-62
2-10 Nomenclature for the Electrostatic Precipitator
Computer Model 2-64
2-11 Typical Electrical Operating Data on Standard
9-inch Plate Precipitator with 0.109-inch Discharge
Electrodes 2-70
2-12 Differences in Particulate Properties in
Conventional and Low-odor Recovery Processes 2-73
2-13 Typical Analysis of Flue Gas from Conventional
and Low-odor Recovery Boilers 2-74
2-14 Typical Operating Conditions in Precipitators
on Bark/Fossil-fuel Boilers 2-78
-------
LIST OF TABLES (continued)
No. Page
2-15 Parameters Affecting Cyclone Design 2-84
2-16 Performance Trends Based on Changes in Cyclone
Design 2-87
2-17 Effects of Physical Properties and Process Variables
on Efficiency 2-88
2-18 Operating Characteristics of Particulate Liquid
Scrubbers on Kraft Lime Kilns 2-89
2-19 Performance Characteristics of Showered Mist
Eliminators on Smelt Dissolving Tanks 2-93
2-20 Design Parameters for Kraft Pulp Power Boiler
Baghouses 2-103
2-21 Process Facilities Controlled by Baghouse Units
Tested 2-105
2-22 Characteristics of Fabric Filter Cleaning Methods 2-109
2-23 Characteristics of Various Fabrics 2-112
2-24 Annual Cost Components for Fabric Filter Control
System 2-118
2-25 Characteristics of Exhaust Gas from Model Sizing
and Transfer Operations 2-120
2-26 Capital and Annual Costs of Fabric Filter Systems
for Model Sizing and Transfer Operations 2-121
2-27 Capital and Annual Costs of Wet Dust-suppression
Systems for Crushers, Screens, Transfer Points,
and Crusher Feeds 2-124
2-28 Capital and Annual Costs of Combination Fabric
Filters and Wet Dust-suppression Systems for
Crushers, Screens, Transfer Points, and Crusher
Feeds 2-126
xi
-------
LIST OF TABLES (continued)
No. Paqe
3-1 Comparison of Design and Operational Features of
Utility and Kraft Pulp Mill Electrostatic
Precipitators 3-10
3-2 Major Maintenance Problems with Utility,
Metallurgical, and Paper Mill Precipitators 3-15
3-3 Scrubber Maintenance 3-31
3-4 Spare Parts Inventory for Venturi Scrubber 3-32
3-5 Manpower Requirements for Maintenance Involving
Plugging and Scaling of Venturi Scrubber 3-33
3-6 Type of Maintenance Required - Venturi Scrubber
Systems 3-34
3-7 Checklist for Routine Inspection of Baghouse 3-47
3-8 Baghouse Collector Maintenance 3-49
3-9 Approximate Cost of Replacement Bags 3-56
3-10 Bag Life in Kraft Pulp Mill and Crushed Stone
Applications 3-56
3-11 List of Replacement Parts for a Baghouse Filter 3-57
4-1 Summary of Inlet Particle Size Distribution Data
Used in ESP and Scrubber Prediction Models 4-2
4-2 Baghouse Particulate Efficiencies - Survey Data 4-24
4-3 Baghouse Particulate Control Efficiencies on
Crushed Stone Industry Processes 4-28
5-1 Evaluation of Dry ESP's for Kraft Pulp Mill
Recovery Boilers and Bark/Fossil-fuel Boilers 5-3
XII
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LIST OF TABLES (continued)
No. Page
5-2 Evaluation of Venturi Scrubbers for Kraft Pulp
Kill Lime Kilns and Crushed Stone Processes 5-4
5-3 Evaluation of Fabric Filters for Kraft Pulp
Mill Bark/Fossil-fuel Boilers and Crushed
Stone Processes 5-5
A.1-1 Selected Research-Cottrell, Inc. Pulp Mill
Recovery Precipitators A-2
A.1-2 Installation List for Wet Scrubbers on Bark
Boilers A-6
A. 1-3 Installation List for Bark/Fossil Fuel Boilers A-7
B.3-1 Troubleshooting Chart for Electrostatic
Precipitators B-40
B.3-2 Frequency of Failure and Repair Times Required
for Various Precipitator Components B-43
Xlll
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METRIC CONVERSION FACTORS
To convert
English units
British thermal unit (Btu)
Cubic foot (ft )
Degrees fahrenheit
Foot
Gallon (U.S. Liquid)
Gallon (U.S. Liquid)
Grain (gr)
Horsepower (hp)
Inch
Inch
Inches of water
Pound
Ton, short
Multiply
by
1054
0.0283
5/9 (°F-32)
0.3048
0.0038
3.7854
0.06479
746.0
0.0254
2.54
248.8
0.4536
0.9C72
Some Common Physical Constants
Boltzmann's constant
Gravitational acceleration
Universal gas constant
Electron volt
Standard temperature =
and pressure (STP)
To obtain
SI units
Joule (j)
Cubic meter (m )
Degrees Celsium (C]
Meter (m)
Cubic meter (m )
Liter (1)
Gram (g)
Watt (w)
Meter (m)
Centimeter (cm)
Pascal (pa)
Kilogram (kg)
Metric ton(kkg)
k = 1.3805 x 10~23 J/°K
g = 9.807 m/s2
R = 8.304 J/mol-°K
eV = 1.602 x 10~19 J
1.013 x 105 Nt/m2
and 273.15°K
XIV
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ACKNOWLEDGMENT
This report was prepared for the Industrial Environmental
Research Laboratory, U.S. Environmental Protection Agency, Re-
search Triangle Park, North Carolina, by PEDCo Environmental,
Inc., of Cincinnati, Ohio; Cottrell Environmental Sciences (CES),
Research-Cottrell, Inc., Somerville, New Jersey; and Midwest
Research Institute, Kansas City, Missouri.
The project director was Mr. Richard W. Gerstle, and the
project manager was Mr. Michael F. Szabo. PEDCo Environmental,
Inc., as the primary contractor and editor, directed and coordi-
nated the project and also provided technical information.
Cottrell Environmental Sciences researched and coordinated
the information on electrostatic precipitators and wet scrubbers.
The work was managed and executed by Mr. David V. Bubenick with
the help of Mr. Chin T. Sui, Dr. P.O. Paranjpe, and Mr. Manuel
Canton. The CES effort was directed by Drs. Paul L. Feldman and.
Richard S. Atkins.
Midwest Research Institute evaluated the fabric filtration
systems. Principal investigator was Mr. Mark A. Golembiewski,
assisted by Mr. V. Ramanathan. Program supervision was provided
by Dr. K.P. Ananth.
xv
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SECTION 1
INTRODUCTION
1.1 PURPOSE OF REPORT
This report is intended to provide guidelines by which en-
vironmental control personnel in the kraft pulp mill and crushed
stone industries can (1) determine which type of particulate
control device is best for a certain process, (2) follow opera-
tional and maintenance practices that will maintain high partic-
ulate collection efficiencies and minimize malfunctions, and (3)
relate the total mass efficiencies of control devices to their
efficiencies for collection of particulate in specific size frac-
tions .
1.2 SIGNIFICANCE OF PARTICULATE EMISSIONS
Many undesirable effects have been related to the discharge
of particulate matter into the atmosphere. Particulates consti-
tute a health hazard, cause poor visibility, function as a trans-
port vehicle for gaseous pollutants, and (in many cases) are
highly active both chemically and catalytically.
The full effects of submicron particulates on health are not
yet well defined. They are regarded as constituting a whole
category of pollutants rather than being a single pollutant.
Once dispersed, they behave (depending on size) similarly to
coarse particles and gases. They remain suspended and diffused,
1-1
-------
are subject to Brownian motion, follow fluid flow around obsta-
cles, and can penetrate deep into the respiratory system.
Particles larger than 5 urn diameter are deposited in the
nasal cavity or nasopharynx. Increasing numbers of smaller
particles, so minute that they are difficult to measure, reach
the lungs. More than 50 percent of particles between 0.01 and
0.1 um that penetrate the pulmonary compartment are deposited in
the lungs. This tendency to penetrate and be captured in the
respiratory system is more a function of the geometry of the
particles than of their chemical properties.
The unhealthy effects of these captured fine particulates
are largely due to their chemical or toxic qualities, although
the physical properties of certain long, fibrous materials may
also irritate tissue. Many unknown factors remain, however, so
it is unwise to generalize concerning the dangers of fine parti-
culates .
1.3 SCOPE OF THE REPORT
This study deals with the following major emission sources
in the kraft pulp and crushed stone industries:
Kraft pulp Crushed stone
Recovery furnaces Crushers
Lime kilns Screens
Smelt dissolving tanks Transfer points
Combination bark/fossil-fuel- Storage bins
fired boilers Drilling equipment
Information presented in this report was obtained from
review of current literature, site visits, and personal communi-
cations with manufacturers and users of control equipment.
1-2
-------
The following high-efficiency particulate control devices
are evaluated: electrostatic precipitators (ESP's), wet scrub-
bers, and fabric filters (baghouses). Cyclone collectors,
although not highly efficient, are evaluated to a limited extent,
because of their historical use as a primary control device and
their present use in controlling coarse particle size distribu-
tions.
In the kraft pulp mill industry, fabric filtration systems
are not used to control particulate emissions from recovery
furnaces, lime kilns, or smelt dissolving tanks. Emissions from
these sources are controlled primarily by wet scrubbers or ESP's.
All four of the control devices are used, however, on combination
bark/fossil-fuel boilers.
In the crushed stone industry, fabric filters are used
almost exclusively for controlling emissions from rock processing
operations. Wet suppression techniques are used either separa-
tely or in conjunction with baghouses to limit fugitive emissions.
Wet scrubbers are also used on stone crushing and conveying
operations.
Section 2 provides descriptions of the subject processes and
discussion of the extent of usage of conventional control devices
to collect their particulate emissions. It then considers
control system design parameters and basic design philosophies.
Some correlations between design parameters are given, as well as
estimates of capital and annual costs of each control device.
1-3
-------
Section 3 describes the operation of each control device and
the maintenance and operational procedures that contribute to
operation at maximum efficiency with minimum downtime. The dis-
cussion encompasses startup, shutdown, normal operational proce-
dures, and common malfunctions.
Fractional collection efficiencies of precipitators, wet
scrubbers, and fabric filters are discussed in Section 4. Com-
puter models are used to predict the fractional efficiency per-
formance of dry precipitators and venturi scrubbers on kraft pulp
mill and crushed stone operations. Almost no test data are
available for comparison with the models. The fractional effi-
ciency relationships of fabric filters are discussed only briefly
because an appropriate computer prediction model is not avail-
able. Only one set of fractional efficiency test data were
available for a mobile fabric filter.
Section 5 presents conclusions on the design, operation,
maintenance, and the fractional efficiency capabilities of the
particulate control devices, including a comparison of the ad-
vantages and disadvantages of applying each type of control
device to the subject industries.
1-4
-------
REFERENCES - SECTION 1
Oglesby, Sabert, Jr. Opening Remarks, EPA/Southern Research
Institute Symposium on Electrostatic Precipitators for the
Control of Fine Particles. EPA-650/2-75-016, Pensacola
Beach, Florida, September 30 - October 2, 1974.
1-5
-------
SECTION 2
CONTROL SYSTEMS: PARAMETERS AND CORRELATIONS
2.1 EMISSION SOURCES, CHARACTERISTICS, AND CONTROL SYSTEMS
Following is a brief discussion of the processes covered in
this report, their particulate emission characteristics, quan-
tities of emissions produced, and the types of conventional con-
trol devices used in each process.
2.1.1 The Kraft Pulping Process
A schematic diagram of the kraft (sulfate) pulping process
is presented in Figure 2-1. The unit operations discussed in
this report are limited to recovery furnaces, smelt dissolving
tanks, and lime kilns. Although not shown in Figure 2-1, the
power boiler is also considered an emission source if it is
fueled (or partially fueled) by wood bark, chips, or coal (re-
ferred to in this report as a "bark/fossil-fuel" boiler).
Pulping is the conversion of fibrous raw materials such as
wood, cotton, or recycled paper into a material suitable for use
in paper, paperboard, or building materials. The principal
source of fibers is wood. In the process, wood chips are cooked
(digested) at an elevated temperature and pressure in "white
liquor," a water solution of sodium sulfide (Na_S) and sodium
hydroxide (NaOH). The white liquor chemically dissolves lignin
(the material that bonds the cellulose fibers together) from the
2-1
-------
Wood Chip*
Steam
Wh;»e L'auor (NaOH
§
O"
_i
u
O
CO
?
o
Evaporator
Gases
5
o
0
Q.
O
LLJ
"o.
'^
3
^
Weak
Block LJQU
S'eam
^~-
Digester
Gases
Blow Tank
elief
4 Puip
t
Pressing
ond Drying
I
i
Vent
Gases
t
Knorter and
Washers
Causticizer
Tank
Settling
Tank
Filter
Fuel
Mud
Calcium
Carbonate
Kiln
Gases
Figure 2-1. Kraft pulping process,
2-2
-------
wood. The remaining cellulose (pulp) is filtered from the spent
cooling liquor, washed with water, and made into paper.
The balance of the process is designed to recover both
cooking chemicals and heat. Spent cooling liquor and the pulp
wash water are combined to form a weak black liquor, which is
concentrated to about 65 percent solids in multiple-effect evapo-
rators, then fired in a recovery furnace. Two main types of
recovery furnace systems are used in the industry: the direct-
contact evaporator system and the newer indirect-contact or "low
odor" system. To minimize total reduced sulfur (TRS) emissions
from the conventional direct-contact system, the concentrated
black liquor must be oxidated before it is combusted in the
recovery furnace. Combustion of the wood lignin dissolved in the
black liquor provides heat for generating process steam and
converting sodium sulfate (Na_S04) to sodium sulfide (Na^S). To
make up for chemicals lost in the operating cycle, salt cake
(sodium sulfate) is usually added to the concentrated black
liquor before it is sprayed into the furnace.
The smelt, consisting of sodium carbonate (Na_CO-J and
sodium sulfide, is dissolved in water to form green liquor, which
is transferred to a causticizing tank, where quicklime (CaO) is
added to convert the sodium carbonate to sodium hydroxide.
Formation of the sodium hydroxide completes the regeneration of
white liquor, which is returned to the digester. A calcium
carbonate mud precipitates from the causticizing tank and is
calcined in a kiln to regenerate quicklime. The condensate
2-3
-------
streams from the digester system and multiple-effect evaporator
system usually contain dissolved TRS gases. These gases may be
removed with a condensate stripping system, using either air or
steam in a stripping column, before these streams are discharged
to the atmosphere.
Recovery Furnaces--
In the recovery furnace, concentrated black liquor is
burned to produce a smelt of sodium carbonate and sodium sulfide,
which is used to reconstitute cooking liquor. Steam is a byprod-
uct of this operation. As shown in Figure 2-2, the furnace
consists of a combustion chamber and heat recovery equipment
(located directly above). The rising flue gases pass through a
superheater section, the boiler tube bank, and an economizer
section before exiting to the contact evaporator unit.
One of the two main types of recovery furnace systems used
in the industry incorporates a direct-contact evaporator in the
final stage of black liquor evaporation; it is called a conven-
tional or direct-contact system. (See Figure 2-3). The other
main type is an indirect-contact, direct-fired, or "low odor"
system. (See Figure 2-4). About 75 percent of the new furnaces
installed in the past 5 years are of this latter design.
Smelt Dissolving Tanks—
The smelt dissolving tank is a large vessel (3000 to 5000
ft ) located below the recovery furnace. A molten mixture,
comprised primarily of sodium sulfide and sodium carbonate
(smelt), is removed continuously removed from the floor of the
2-4
-------
BABCOCK & WILCOX
Steam
COMBUSTION ENGINEERING
4 Steam
LEGEND
1 . Furnace
2. Smelt Spouts
3. Block Liquor
4. Primary Air Supply
5. Secondary Air Sopoly
6. Tertiary Air Supply
7. Position of Char Bed Burner? for Oil or Gas
8. Normal Configuration o' Char Bed
8'. Same at Lav. Primary Air Flow and Pressure
9. Screen Tubes
10. Superheater
1 1 . Boiler Tube Bonk
12. Exit to Economizer
s r
Section A - A
Figure 2-2. Principal design of recovery boiler furnace.
2-5
-------
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2-7
-------
recovery furnace. Water is added to this molten mixture in the
smelt dissolving tank to produce green liquor. This tank is an
open vessel equipped with an agitator to assist dissolution and
a steam or liquid shatterjet system to break up the smelt stream
before it enters solution. Entrained particulates are emitted
with large volumes of steam as the molten smelt and water mix.
The smelt dissolving tank is one of the main sources of partic-
ulate matter in a kraft pulp mill.
Lime Kilns--
The lime kiln is an essential element of the closed-loop
system that converts the green liquor solution of sodium carbon-
ate and sodium sulfide to white liquor. In the kiln the lime
mud (calcium carbonate that precipitates from the causticizer)
is calcined to produce calcium oxide (quicklime, CaO) for re-
causticizing the green liquor. The lime sludge typically enters
as a slurry with 55 to 60 percent solids.
The kraft pulping industry generally uses large rotary
kilns capable of producing 40 to 400 tons/day of quicklime.
Fluidized-bed calciners are being used at four pulp mills, but
their current production rate is less than 150 tons/day and
accounts for only about 1 percent of the total quicklime produced
in the kraft industry.
The rotary kiln turns at low speeds, causing the lime to
proceed downward toward the high-temperature zone (1800° to
2000°F), which is sustained by combustion of natural gas or fuel
oil. The lime mud dries as it moves along (often aided by
2-8
-------
chains). Baffles in the upper section of the kiln are sometimes
used to provide better contact with the hot gases. Near the
lower end of the kiln, the lime agglomerates into small pellets,
and finally is calcined to calcium oxide in the high-temperature
zone near the burner.
These lime kilns differ from those used in the lime manufac-
turing industry in that the calcium carbonate is generally fed
as a mud (sludge) containing 40 to 45 percent water instead of
as a solid (limestone). This sludge contains a small percentage
of sodium sulfide, which affects the size distribution and
composition of the particulate in the exhaust gases. The lime-
stone used in the lime industry does not contain sodium sulfide.
Bark/Fossil-Fuel Power Boilers--
Based on 1972 figures compiled by the American Paper Insti-
tute, about 7 percent of the total energy requirement of the
pulp and paper industry is supplied by combustion of wastewood
and bark, an additional 33 percent is supplied by combustion of
its waste pulping liquors, and the remainder is provided by
2
fossil fuels or by purchased electricity.
According to a 1970 survey, 32 percent of the reporting
pulp and paper mills used bark plus other fuels in their power
boilers. On a Btu basis, the average fuel utilization for a
group of 26 mills that reported emission data was as follows:
bark/wood, 48.5 percent; oil, 31.0 percent; coal, 11.3 percent/-
and gas, 9.2 percent.
Wastewood- and bark-fired power boilers can burn wood alone
2-9
-------
or can be modified to burn other fuels on an auxiliary basis or
in combination. In power boilers, wastewood and bark are
burned on chain grates in a radiant Dutch-oven-type boiler or in
a horizontal, air-blown, suspended firing configuration in a
vertical Stirling boiler. Wood handling systems (including
harranerrni 11 grinding to a given particle size for suspended
firing), bottom and fly ash handling systems, and underfire and
overfire air controls must be provided. Major fuel characteris-
tics affecting the design of wastewood-fired power boilers
include ash and moisture content, particle size variations, and
fixed and volatile carbon content.
The net heating value of most wastewoods is about 7200 to
9000 Btu/lb of dry wood or 3600 to 5400 Btu/lb on an as-fired
basis. The heating value of wastewoods tends to vary with wood
species. The presence of extractive materials such as terpenes
and tall oils can substantially add to the energy content of the
wood.
2.1.2 Emission Characteristics - Kraft Pulping
Characteristics of emissions from the various sources
discussed above are summarized in Tables 2-1 and 2-2. The
average U.S. pulp mill emits about 5.5 Ib of particulate per ton
of air-dried pulp (ADP). A well-controlled mill emits about 2.8
Ib/ton ADP. Nationwide, particulate emissions from kraft pulp
mills run about 89,000 tons/yr. This amount would be reduced by
about 49 percent if the best available control systems were
applied to recovery furnaces, lime kilns, and smelt dissolving
2-10
-------
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2-11
-------
Table 2-2. CHARACTERISTICS OF PARTICULATE EMISSIONS
FROM SELECTED U.S. POWER BOILERS3
Number
of
boilers
18
2
2
16
2
2
2
3
2
Percent of fuel supplied
(3tu basis)
Coal
100
100
100
0
75
0
73
0
0
Oil
0
0
0
46
0
0
16
25
0
Gas
0
0
0
0
0
62
0
39
0
B/W"
0
0
0
54
25
38
11
36
100
Particulate
concentration
(uncontrol led ) ,
gr/ft3
1.87
4.89
3.47
2.30
1.88
1.40
E/W = bark and wastewood.
2-12
-------
tanks.
Except for those from power boilers, particle size data are
generally lacking on emissions from kraft pulping operations.
Uncontrolled emissions from each process are discussed next.
Recovery Furnaces--
The flue gas from black liquor recovery boilers contains
considerable particulate matter, which is formed by the release
of sodium from the smelt bed to the flue gas above it. The
amount of sodium released appears to depend not on the sodium
content of the black liquor dry solids, but rather on partial
vapor pressures and diffusion conditions. Characteristics of
emissions from conventional versus low order recovery furnaces
are discussed below.
The primary difference in the low odor recovery furnace
operation as compared to the conventional process is the eli-
mination of direct contact between the concentrated black liquor
and the recovery boiler gases. In general, the black liquor
leaving the evaporator at 45-50 percent solids is further con-
densed to 60-65 percent solids is a concentrator. It is then
piped directly to the recovery boiler.
Particulate emissions from a conventional recovery furnace
prior to application of a direct-contact evaporator or a control
device normally range from 8 to 12 gr/dscf (200 to 450 Ib/ton
ADP). A direct-contact evaporator acts as a particulate control
device and reduces particulate emissions from a furnace system by
about 50 percent. Particulate emissions from uncontrolled
2-13
-------
conventional recovery furnace systems currently in operation
average about 3.81 gr/dscf versus about 7.93 gr/dscf for low
odor boilers. Particulate matter emitted from the recovery
furnace consists of sodium sulfate and sodium carbonate and may
contain small amounts of sodium chloride. Sodium chloride will
be present if the pulpwood has been stored in saline water or if
the makeup chemicals contain chloride impurities.
Flue gas volume flow rates from recovery furnaces in general
can range from 32,000 to 400,000 scfm depending on pulp produc-
tion rate and volume of excess air used. Temperatures of the
exiting gases (after economizer) fall in the range of 250° to
360°F for the conventional process versus 340°-450°F for the low
odor process.
The conventional kraft recovery furnace is also one of the
largest emitters of reduced sulfur compounds in the pulping
process. Concentrations of sulfur dioxide (S0_) in the furnace
exhaust gases may be as high as 880 ppm.
Smelt Dissolving Tanks—
Particulate consisting of dissolved and undissolved NaOH,
Na.CO,, and Na S is emitted from the dissolving tank with the
vented gases. Uncontrolled emissions from a typical tank (1000
tons of pulp/day) may be as high as 380 Ib/h (8.0 Ib/ton ADP).
Exhaust gas flow rates may range from 11,000 to 22,000 scfm at
160° to 230°F.
Lime Kilns—
Lime kiln particulate emissions consist principally of
sodium salts, calcium carbonate, and calcium oxide. Because the
2-14
-------
sodium salt (soda fume) emissions result primarily from sodium
compounds that are retained in the mud because of inefficient or
incomplete washing, particulate emissions are affected by the
efficiency of the mud washing system (higher than normal sodium
levels in mud result in higher particulate emissions). The
calcium particles result from entrainment; thus, emissions are
affected by the gas velocity and turbulence in the kiln. Uncon-
trolled particulate emissions from a typical lime kiln can range
from 3 to 15 gr/scf, with exhaust volumes of 15,000 to 66,000
scfm. Gas temperatures are usually about 400° to 500°F. The
dust losses constitute about 1 to 5 percent of the total dry
solids load to the kiln.
The rolling and tumbling of the lime in the rotary kiln and
the vaporization of sodium compounds (carried into the kiln with
the lime mud) in the high-temperature zone and their later con-
densation are responsible for formation of most of the partic-
ulate matter in the kiln exhaust gases. In addition to being an
air pollution problem, these emissions constitute a loss of
usable chemicals.
The lime dust, made up of particles ranging from 1 to over
100 ym in diameter, is removed from the exhaust gas quite easily,
The soda fume consists of very small particles (most less than
1 ym in diameter) and is very difficult to remove.
Bark/Fossil-fuel Boilers--
The major potential air pollutant from wood-fired power
boilers in the pulp and paper industry is particulate matter,
2-15
-------
which can result either from inorganic ash in the wood or from
incomplete combustion.
Particulate emissions from wastewood combustion vary with
ash content and particle size of the material being burned. The
ash content can vary from less than 1 to 20 percent by weight on
an as-fired basis. The sizes and shapes of the wood particles
being burned can influence the design of grating systems, the
kind of firing, and the relative distribution of underfire and
overfire air in the furnace. Furnace fouling often occurs when
bark is fired, especially when it is fired with other fuels.
Small amounts of minerals that are gathered in the fuel while it
is stored in sea water or on ground can lower the ash softening
temperature so that it becomes very sticky in the furnace and
cannot be easily removed. The amount of particulate matter swept
from the combustion chamber is normally greater from horizontal
suspension firing units than from Dutch-oven-type units.
Characteristic uncontrolled rates of particulate emission
from pulp mill power boilers were shown previously in Table 2-2.
Emissions from boilers firing bark or a combination of bark and
fossil fuels range from 1.4 to 3.5 gr/scf. Typical gas flow
rates in these types of boilers range from 19,000 to 200,000 scfm
at temperatures around 450°F.
Particle size characteristics of emissions from bark/fossil-
fuel boilers are presented in Table 2-3. Bark that has been
soaked in salt water contains a much greater percentage of fine
particles than does freshwater bark (60 to 70% particulate less
2-16
-------
Table 2-3.
TYPICAL PARTICLE SIZE DISTRIBUTION OF FLY ASH
FROH BARK/FOSSIL-FUEL BOILERS3
A.
Boiler fired with 100% bark
Particle size range,
;jm
<5
5-10
10 - 20
20 - 50
50 - 104
104 - 147
147 - 175
175 - 590
-590
Total
Other0
Percentage by weight
Flue gas
19.76
11.56
8.67
4. 82
3.37
16.24
3.65
17. 09
12.77
97. 93
2.07
Stack gas after cyclone
38.93
16.57
13.25
8.28
5.80
9.05
1.54
2.71
1.25
97. 38
2.62
B.
Boiler fired with 401 bark - 60% coal
Particle size range,
;jm
+ 60
40-60
30-40
20-30
15-20
10-15
7.5-10
5. 0-7.5
3.5-5.0
2. 5-3.5
1.5-2.5
-1.5
Total %
% <10 u
% by
8.
7.
8.
13.
9.
14.
9.
10.
6.
5.
5.
4.
100.
31.
weight
5
5
0
0
0
0
0
0
5
0
0
5
0
0
Particle size range by Banco method assuming a spherical particle
and a specific gravity of 2.5.
Average of two sampling runs.
Due to handling loss.
2-17
-------
than 1.0 nn in diameter in high salt fuel). Potential gaseous
emissions a: oxides of nitrogen, oxides of sulfur, and hydro-
carbons resulting from volatilization of the bark. The nitrogen
oxide emissions are generally lower than those from fossil-fuel-
fired boilers because of the larger combustion volumes per unit
amount of fuel burned, the normally higher excess air levels (50
percent or more), and the higher fuel moisture content that
results in low flame temperatures of 1800 to 2200°F. Emissions
of SO from bark-fired boilers generally are low because the
sulfur content of bark is normally less than 0.1 percent by
weight. Emissions of terpenes, hydrocarbons, and other volatile
organic constituents as a result of distillation and incomplete
combustion vary with wood species, furnace temperature, and
retention time. The potential of these emissions as air pollu-
tants has not been fully described.
2.1.3 Control Methods - Kraft Pulping
The methods of emission control used at kraft pulp mills are
presented in the following paragraphs.
Recovery Furnaces--
Nearly all recovery furnaces are equipped with electrostatic
precipitators for primary particulate control. The degree of
control provided, however, varies among the individual units.
Design efficiencies range from about 90 percent on older precipi-
tators to above 99.5 percent on recent installations. The lower
efficiencies on older units were established by balancing the
value of the recovered soda ash to the capital outlay and oper-
ating expenses of the ESP.
2-18
-------
Until recently, almost all recovery furnace systems incorpo-
rated a direct-contact evaporator. Although the purpose of the
evaporator is to concentrate black liquor, it also scrubs partic-
ulate matter from the gas stream. Depending on the type of
direct-contact evaporator used, it can remove up to 50 percent of
the particulate.
Most mills use direct-contact evaporators of the cascade
type. The furnace gases pass over a trough filled with black
liquor, which is scooped up by a rotating paddle wheel and then
cascaded through the gas stream. Some mills use cyclones or
Venturis as the direct-contact evaporator. In these installa-
tions the black liquor serves as the particulate scrubbing
liquid. Sometimes two Venturis are used in series to increase
particulate collection, precluding the need for an electrostatic
precipitator.
On some recovery furnaces, scrubbers have been installed
downstream from the precipitators. 'In the United States this
practice has been confined to upgrading of existing units.
No applications of fabric filtration have been reported for
particulate emission control on recovery furnaces. Because the
emitted matter is sticky, the filter bags would quickly become
blinded. Mechanical dust collectors are also unsuitable for this
application because of the small particle size of the particulate.
Information on Research Cottrell precipitators on kraft pulp
mill recovery boilers is presented in Appendix A-l.
2-19
-------
Smelt Dissolving Tanks--
The gases from most smelt dissolving tanks are vented
through mist eliminator pads with fine wire mesh screens about 1
ft thick. Mist eliminator pads are basically low-energy scrubbers
with collection efficiencies of about 80 percent. Droplets
condensing from the gas collect on the screen and are back-
flushed with water sprays to the dissolving tank. Several
dissolving tanks are equipped with more efficient water scrubbers,
such as low-pressure drop Venturis (6 to 8 in. of water), packed
towers, and cyclones with water sprays. Efficiencies of these
systems run about 95 percent. A few mills combine the dissolving
tank gases with the recovery furnace gases and send both streams
to an electrostatic precipitator.
Lime Kilns--
The major ways of controlling particulate emissions from
lime kiln and fluidized-bed calciner exhaust gases are liquid
scrubbing (using either an impingement or a venturi-type scrub-
ber) and recently, electrostatic precipitation. Scrubbing
devices are usually located after a mechanical cyclone collector.
Most lime kilns at kraft pulping plants are controlled with
venturi scrubbers with pressure drops ranging from 10 to 25 in.
H_0. These systems provide collection efficiencies of up to
about 99 percent. A few kilns are controlled by impingement
scrubbers with wetted baffles and water sprays. These scrubbers
have pressure drops of 5 to 6 in. HO and provide collection
efficiencies of only about 90 percent.
2-20
-------
Some kilns in Sweden are controlled by electrostatic pre-
cipitators. Design efficiencies are about 99 percent. One U.S.
mill has a retrofitted precipitator serving three kilns.
Fabric filtration is not used as a control method for lime
kiln emissions because of the moisture content (25 to 35%) of the
exhaust gases.
Bark/Fossil-fuel Boilers--
Control of particulate emissions from wood and bark/fossil-
fuel boilers normally is not as complex as control of emissions
from other processes within the pulp and paper industry. The
problems and solutions are similar to those in conventional
boilers.
The particulates from bark firing are often large, 5 to 10
ym or greter. Because their specific gravity is usually low,
the use of mechanical cyclone collectors is not always effective.
Electrostatic precipitation of the fly ash is difficult because
the high carbon content can cause low particle resistivity.
Minerals in the bark can cause abrasion in the collecting equip-
ment and ducts and resultant rapid metal wear. More complete
analysis and classification of the chemical composition and
physical size characteristics of particulate matter emitted
from bark-fired power boilers are needed.
Electrostatic precipitators have been installed on only a
few bark/fossil fuel fired boilers but have shown collection
efficiencies as high as 99.9 percent, (bark/coal fired boiler).
Cyclone collectors offer less efficient, but also less expensive,
2-21
-------
control for bark/fossil fuel boilers, and wet scrubbers or fabric
filters (which are effective for removal of particles below 5 urn)
offer an alternative to electrostatic precipitation.
Granular bed scrubbers have also reportedly been used suc-
cessfully on a number of non-salt hogged fuel boilers. However,
efficiency of the granular bed scrubbers falls off rapidly for
particles less than about 2 micrometers in diameter. The Simpson
Timber Co. in Shelton, Washington rejected the granular bed
scrubber for use in controlling salt laden particulate from its
hogged fuel boilers because the scrubber would not remove the
blue haze caused by the primarily sub-micron salt particles.
Since the granular bed scrubber is not an efficient collect-
or of fine particles, and has not been fully developed, a detailed
discussion of this device is not included in this report.
Two pulp mills in the State of Washington use fabric filtra-
tion systems to control emissions from their power boilers. Both
fire 100 percent bark and wastewood.
The Simpson Timber Company in Shelton u as two fabric fil-
ters to reduce particulate emissions from their two power boilers.
Both are equipped with Teflon B-coated fiberglass bags. The
units handle 100,000 and 130,000 acfm at about 500°F. Both
collectors are preceded by mechanical collectors and were; in-
stalled to remove submicron NaCl particles from the exhaust gases
because the plant had experienced opacity problems with the
plumes from the power boiler stacks. The air-to-cloth ratio
(A/C) is 4.5/1; the pressure drop is about 9 in. H^O. The
2-22
-------
collectors are cleaned by pulse-air action, and operating exper-
ience is described by plant personnel as reasonably good in the
2 years since installation.
The Long Lake Lumber Company in Spokane is the other pulp
mill using fabric filtration for power boiler emission control.
The fabric filter cleans approximately 17,200 acfm of effluent at
400°F and is equipped with Nomex bags that are cleaned by the
pulse-jet method. A/C ratio is 4.0/1. This collector has
reportedly been operating well for 3 years at a collection
efficiency of 99.7 percent.
A Georgia-Pacific mill in Bellingham, Washington, has
recently contracted Standard Havens to install a 180,000-acfm
fabric filter to control particulate emissions from its four
bark-fired power boilers. The unit will reportedly operate at a
net A/C ratio of 4.0/1 and a temperature of about 440°F. It will
be outfitted with Teflon-coated fiberglass bags. Georgia-Pacific
selected a fabric filtration rather than a dry gravel bed system
and venturi scrubber because they believe it to be the most
reliable and proven means of control.
Two recent bark/oil-fired boilers equipped with venturi
scrubbers are Crown Zellerbach's Port Townsend and Port Angeles
plants near Seattle. The Port Townsend plant has a salt emissions
problem because it fires sea-soaked bark; the Port Angeles plant
does not. Both scrubbers are located downstream from a multi-
clone collector. The Port Townsend scrubber operates at pressure
drops ranging from 15 to 20 in. HO, and outlet emissions have
2-23
-------
been tested at 0.07 to 0.18 gr/dscf depending on the salt content
of the bark. Sodium chloride accounts for 26 to 70 percent of
total emissions. The Port Angeles scrubber operates at pressure
drops from 8 to 10 in. H^O and outlet emissions have been measured
at 0.022 gr/dscf. Thus, the Port Angeles scrubber achieves a
lower emission rate at a lower pressure drop than the Port Town-
send scrubber. Opacity of the Port Angeles stack also is very
low. These differences in operation can be attributed in large
part to the salt content of the hogged fuel burned at the Port
Townsend plant, which cannot meet the 0.10 gr/dscf particulate
emission regulation when the salt content of the fuel is greater
than 1 percent.
A listing of bark/fossil-fuel fired boilers from the National
Emission Data System (NEDS) is presented in Table A.1-2, Appendix
A. A number of particulate scrubbers operating on bark/fossil-
fuel boilers are summarized in Table A.1-3, Appendix A-l.
2.1.4 Crushed Stone Processing
The conversion of naturally occurring minerals into crushed
stone products involves a series of interrelated physical opera-
tions. Quarrying, transporting, crushing, size classification,
and drying are common to almost all methods of mineral produc-
tion. Particulate air pollution may result from any or all of
these operations. The dust emitted is usually a heavy partic-
4
ulate released at ambient temperature.
The initial step in processing of crushed stone occurs at
the quarry site. Blast holes are drilled vertically into the
exposed stone faces and charged with explosives; the rock is then
2-24
-------
blasted out of its deposit. If secondary breakage is required,
"drop ball" cranes are normally used. Oversize rock may also be
reblasted. The broken rock is transported by 35- to 50-ton
trucks from the quarry pit to the primary crusher, which is often
in or near the quarry. When portable plants are located in the
quarry itself, material can be fed directly to the primary
crusher by a power shovel.
Crushing plant operations common to most crushed stone
installations are primary crushing, scalping, secondary crushing,
tertiary or finishing crushing, final screening, conveying,
storage and shipping, and in some instances, washing. Depending
on the purpose of the plant and the kind of rock processed, all
or only a few of these operations take place.
As illustrated in the flowsheet in Figure 2-5, broken rock
obtained from the quarry is dumped into a hoppered feeder,
usually a vibrating grizzly type, and fed to the primary crusher
for initial reduction. Jaw or gyratory crushers are normally
used, although impact crushers are gaining favor for crushing
low-abrasion rock (e.g., limestone) and when high reduction
ratios are desired. The crusher product (normally in pieces 3 to
12 in. in size) and the "grizzly throughs" are discharged onto a
belt conveyor and transported to a surge pile or silo for tem-
porary storage.
The material is then reclaimed, usually by a series of
vibrating feeders under the surge pile, and conveyed to a scalp-
ing screen, which separates the process flow into three fractions
2-25
-------
c
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cu
c
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2-26
-------
(oversize, undersize, and "throughs") prior to secondary crush-
ing. The oversize is discharged to the secondary crusher for
further reduction. The undersize, which requires no further
reduction at this stage, normally bypasses the secondary crusher,
thus reducing its crushing load. The "throughs," which contain
unwanted fines and screenings, are usually removed from the
process flow and stockpiled as crusher-run material. Gyratory or
cone crushers are the most commonly used for secondary crushing,
although impact crushers are used at some installations.
At a typical operation the product from the secondary
crushing stage (usually 1 in. or less in size) is transported to
a secondary screen for further sizing. Sized material from this
screen is conveyed or discharged directly to tertiary crushing,
which takes place on cone crushers or hammermills. The product
from the tertiary crushers is shuttled back to the secondary
screen, forming a closed circuit with a fixed top size. The
throughs from this screen are discharged to a conveyor and
elevated to a screen house or tower containing multiple-screen
lines for final sizing. At this point, end products of desired
gradation are chuted directly to finished product bins or trans-
ported by conveyors or trucks to stockpiles in open areas.
Sometimes stone washing is required to meet particular end
product specifications or demands, such as those for concrete
aggregate. Washing plants consist of a number of fine mesh
screens onto which the material falls and is sprayed with a heavy
water-spray. Unwanted fines are usually discharged to a settling
pond.
2-27
-------
2.1.5 Emissions Characteristics - Crushed Stone
Particulates emanate from many sources (both process and
fugitive) in a quarry and crushed stone plant. Process sources
include drilling, crushing and grinding, conveying and elevating
(transfer points), stockpiling (the actual operation itself),
and screening. Fugitive emissions are generated by blasting,
loading, hauling, stockpiling (e.g., free fall), and also are
windblown from roads, plant yards, and stockpiles. No precise
estimate of fugitive emissions has been made.
Quantitative data on emissions from these sources are
practically nonexistent. The few data available are conflicting
and, in some cases (such as when material balances are used)
without basis. Examples of the ranges in the available emissions
data and corresponding emission factors are presented in Table
2-4.
Examination of the data in Table 2-4 leads to several
conclusions. First, emission factors do not take into account
the kind of rock, which is a very important parameter. Second,
in most cases reported emissions are considerably higher than
the emission factors. Third, reported emission values are
inconsistent with the operation performed and the corresponding
emission factor. For example, the emission factor for secondary
crushing and screening is three times that for primary crushing,
but reported emissions from primary crushing are much higher
than those for secondary.
Factors affecting emissions that are common to most crushed
2-28
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Table 2-4. REPORTED UNCONTROLLED EMISSIONS VERSUS
EMISSION FACTORS^
Operation
Primary crushing
Secondary crushing and
screening
Tertiary crushing and
screening
Recrushing and
screening
Screening, conveying,
and handling
Reported emissions,
Ib/ton
4.9 - 120
0.26 - 13.7
3.6 - 18.2
0.6 - 7. 97
1.4 - 125
Emission factors,
Ib/ton
0.5
1.5
6. 0
5. 0
2. 0
2-29
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stone operations include moisture content of the rock, the kind
and amount of rock processed, the equipment and operating prac-
tices, and a variety of geographical and seasonal factors..
These factors apply to both fugitive and process sources asso-
ciated with either quarrying or plant operations.
.Minimal data are available to define the particle size
characteristics of dust generated in stone processing operations.
It is usually a fairly coarse particulate containing some mois-
ture. Particle size depends on the kind of rock, the processing
equipment, and the stage of processing. Figure 2-6 presents
particle size data on emissions from a jaw crusher and a conveyor/
screening operation. Less than 10 percent of the particles
emitted from the crusher are smaller than 10 pm, whereas 50
percent of those conveyor/screening operations are below 10 urn.
Drilling (Quarrying)—
Emissions from drilling operations result primarily from
air flushing to remove cuttings and dust from the bottom of the
hole. Putting compressed air down the hollow drill center
forces cuttings and dust up and out the annular space between
the hole wall and drill. Factors affecting the level of uncon-
trolled emissions include the kind of rock, the moisture content
of the rock, the type of drill, the diameter of the hole, and the
penetration rate.
Crushing—
Generation of particulate emissions is inherent in the
crushing process. Most apparent at crusher feed and discharge
2-30
-------
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2-31
-------
point, the emissions are influenced by various factors, including
the moisture content of the rock, the kind of rock, the pro-
cesses, and the type of crusher.
The crushing mechanism (i.e. compression or impact) is the
most important element influencing emissions from crushing
equipment. It has a substantial effect on the size reduction
achieved; the particle size distribution of the product, es-
pecially the proportion of fines produced; and the amount of
mechanically induced energy that is imparted to the fines.
Impact crushers produce a larger proportion of fines than
compression crushers. They also impart more velocity to the
fines as a result of the fan-like action produced by the spinning
hammers. Hence, impact crushers generate more uncontrolled
particulate emissions per ton of stone processed than any other
crusher type.
Uncontrolled emissions from compression crushers (jaw,
gyratory, cone, and roll crushers) closely parallel the reduction
stage to which they are applied; the greater the reduction, the
higher the emissions. Primary jaw crushers probably produce more
dust than comparable gyratories because of the bellows effect of
the jaw and because gyratory crushers are usually choke-fed,
which minimizes the open spaces from which dust can be emitted..
In subsequent reduction stages, cone crushers produce more fines
as a result of attrition, and consequently generate more dust.
Screening—
Dust is emitted from screening operations as a result of
2-32
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the agitation of dry stone. The level of uncontrolled emissions
is dependent on the particle size of the material screened, the
amount of mechanically induced energy transmitted, and other
factors already discussed.
Generally, the screening of fines (less than 1/8 in.) pro-
duces more emissions than the screening of coarse sizes. Screens
agitated at large amplitudes and high frequency emit more dust
that those operated at small amplitudes and low frequencies.
Conveying (Transfer Points)--
Particulates may be emitted from any and all materials
handling and transfer operations. As with screening, the level
of uncontrolled emissions is dependent on the size of the mate-
rial and how much it is agitated. Perhaps the worst case
occurs at conveyor belt transfer points, where material is
discharged from the conveyor at the head pulley or received at
the tail pulley. The quantity of emissions depends on the con-
veyor belt speed and the free-fall distance between transfer
points.
Storage Bins--
The transfer of final stone product to storage bins by
conveyor belt or chute generates dust emissions similar to those
from other transfer operations. Significant particulate emis-
sions also may evolve from loadout of the stored material into
open dump trucks. Again, the amount of dust generated depends
on the moisture content of the stone and the free-fall distance.
2-33
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2.1.6 Control Methods - Crushed Stone Operations
Because a typical stone crushing plant contains a multi-
plicity of dust-producing points, effective emission control is a
complex and difficult problem. Methods for control of plant-
generated emissions include wet scrubbers, wet dust suppression,
dry collection, and a combination of both wet suppression arid dry
collection. In wet dust suppression, moisture is introduced into
the material flow, causing fine particulate matter to be confined
and remain with the material flow rather than to become airborne.
Dry collection involves hooding and enclosing dust-producing
points and exhausting emissions to a collection device. Combina-
tion systems (Figure 2-7) utilize both methods at different
stages throughout a stone processing plant. The use of enclosed
structures to house process equipment is also an effective control
technique.
In a wet dust-suppression system, dust emissions are con-
trolled by spraying moisture (water or water plus a wetting
agent) at critical dust-producing points in the process flow.
This causes dust particles to adhere to larger stone surfaces or
to form agglomerates too heavy to become, or remain, airborne.
Thus, the objective of wet dust suppression is not to fog an
emission source with a fine mist to capture and remove emitted
particulates, but rather to prevent their emission by keeping the
material moist at all process stages.
Small quantities of specially formulated wetting agents or
surfactants are blended with water to reduce its surface tension
and consequently improve its wetting efficiency so that dust
particles may be suppressed with a minimum of added moisture.
2-34
-------
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The dilution of such an agent in minute quantities in water
(1 part wetting agent to 1000 parts water) is reported to make
dust control practical throughout an entire crushed stone plant
with as little as 1/2 to 1 percent total added moisture per ton
of stone processed.
A typical wet dust-suppression system illustrated in Figure
2-8, contains several basic components and features, including
(1) a dust control agent (compound M-R), (2) proportioning
equipment, (3) a distribution system, and (4) control actuators.
A proportioner and pump are necessary to proportion the wetting
agent and water at the desired ratio and to provide the moisture
in sufficient quantity and at adequate pressure to meet the
demands of the overall system.
Distribution is accomplished by spray headers fitted with
pressure spray nozzles. Headers are used to apply the dust-
suppressant mixture at each treatment point at the rate and spray
configuration required to effect dust control.
Particulate emissions generated at plant process facilities
(crushers, screens, conveyor transfer points, and bins) may also
be controlled by capturing and exhausting emissions to a dry
collection device. Depending on the physical layout of the
plant, emission sources may be manifolded to a single centrally
located collector or to a number of strategically placed units.
Collection systems consist of an exhaust system with hoods and
enclosures to confine and capture emissions and ducting and fans
2-36
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Truck Dump
and Feeder
Bog Collector
V
Primary
Crusher Secondary
0 Crusher
Bin and Truck
Loadinc Station
Succressior
Collecrior
Tertiary
Crusher
Figure 2-8. Wet dust-suppression systems.
2-37
-------
to convey the captured emissions to a collection device where
particulates are removed from the air stream before it is ex-
hausted to the atmosphere.
The most commonly used dust-collection device in the crushed
stone industry is the fabric filter (or baghouse). In most
crushing plant applications, mechanical-shaker collectors (which
require periodic shutdown for cleaning after 4 or 5 hours of
operation) are used. These units are normally equipped with
cotton sateen bags and operated at an A/C ratio of 2 or 3 to 1.
A cleaning cycle, normally actuated automatically when the
exhaust fan is turned off, usually requires only 2 or 3 minutes
of bag shaking.
Fabric filters with continuous cleaning are used where it
may be impractical to turn off the collector. Compartmented
mechanical-shaker units may be used, but jet-pulse units are
preferred. Jet-pulse units normally have wool or synthetic
felted bags as the filtering medium and can be operated at a
higher filtering ratio (as high as 6 or 10:1). Greater than 99
percent efficiency can be attained with either type fabric
filter, even on submicron particle sizes. Outlet grain loadings
recorded during EPA emission tests at several crushed stone
facilities processing various kinds of rock seldom exceeded
0.01 gr/dscf.5
Other collection devices reportedly used in the industry
include cyclones and low-energy scrubbers. Although these
collectors may provide high efficiencies (95 to 99%) on coarse
particles (40 um and larger), their efficiencies are poor (less
2-38
-------
than 85%) on medium and fine particles (20 \im and smaller).
Although high-energy scrubbers and electrostatic precipitators
could conceivably achieve results similar to those of a fabric
filter, these methods are not currently used in the industry.
Dust control at portable crushed stone plants is considered
by some industry spokesmen to be extremely difficult; however,
the successful application of a wet dust-suppression system has
been reported. Furthermore, trailer-mounted, portable baghouse
units are commercially available and have been used to control
emissions from portable asphalt concrete batch plants. Although
application of dry collection systems at portable crushed stone
plants is not widespread, portable plant equipment manufacturers
have indicated unofficially that this control option is indeed
feasible and that the required hoods, enclosures, and ductwork
could be integrated into the design of portable plant components.
At least one manufacturer has drafted a proposal for such an
installation.
Drilling--
The two control methods available for controlling particulate
emissions from drilling operations are water injection and
aspiration to a control device.
Water injection is a wet drilling technique in which water
or water plus a wetting agent or surfactant, usually a liquid
detergent, is injected into the compressed air stream used to
flush the drill cuttings from the hole. The injection of the
fluid into the air stream produces a mist, which dampens the
2-39
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stone particles and causes them to agglomerate. As the particles
are blown from the hole, they drop as damp pellets at the drill
collar rather than becoming airborne.
Dry collection systems can also be used to control emissions
from the drilling process. A shroud or hood encircles the drill
rod at the hole collar. Emissions are captured under vacuum and
vented through a flexible duct to a control device for collection.
The device is often mounted on the drilling rig. Various devices
are available, and their efficiencies vary. Cyclones or fabric
filters preceded by a settling chamber are the most common. Air
volumes required for effective control range from 500 to 1500
cfm, depending on the kind of rock, the hole size, and the
penetration rate.
Crushing—
Wet suppression techniques are currently used on most
crushers, with the exception of limestone crushers. Application
of treated water at the feed point of the crusher has been proven
effective in minimizing particulate emissions.
When dry collection systems are used, the upper portion of
the crusher should be enclosed as completely as possible and
exhausted according to the criterion established for transfer
points. The crusher discharge/transfer belt interface also
should be totally enclosed. An example of this type of system
is shown in Figure 2-9. The exhaust rate will vary considerably
with the type of crusher. Exhaust volumes from impact crushers
or hammermills range from 4000 to 8000 cfm, whereas an exhaust
2-40
-------
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2-41
-------
rate of 500 cfm/ft of discharge opening should be sufficient on
compression-type crushers. In either case, pickup should be
applied downstream of the crusher at a distance at least 3.5
times the width of the receiving conveyor.
To achieve effective emission control, ventilation should
be applied both at the upper portion (or feed) of the crusher
and at the point of transfer to the belt (or crusher discharge).
This would not apply to primary jaw or gyratory crushers because
operators must have ready access to dislodge large rocks that
sometimes become stuck in the crusher feed opening.
Conveyor Transfer Points--
Water spraying effectively controls the dust generated by
transferring crushed stone between conveyors. This method
cannot be used, however, when a dry product is needed at the
next step of the operation or when there is a market for the
fines collected by a dry control system.
The alternative method of control is complete enclosure of
the conveyor or the transfer point. Complete enclosure of both
is impractical and rarely used.
At belt-to-belt conveyor transfer points, hoods should be
designed to enclose both the head pulley of the upper belt and
the tail pulley of the lower belt as completely as possible. By
careful design, the open area should be reduced to about 0.5
ft /ft of belt width. Conveyor belt speed and free-fall
distance are factors that affect the air volume to be exhausted.
2-42
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Screening--
Water sprays are used in most operations where wet or damp
stone can be tolerated in the rest of the process. Water spray
bars or nozzles are normally located at the discharge point of
the conveyor, on the screen itself, and at the discharge points
of the screen. This method has the following disadvantages: wet
dust may bind on small screens, the water may retard later
processes or prevent the stone from meeting specifications if the
dust is not washed off. The use of washing plants will overcome
most of these disadvantages.
Effective dry control at screening operations normally re-
quires the use of several exhaust points. A full-coverage hood
is often used to control emissions generated at screening sur-
faces. Additional or alternative ventilation air may be required
at the screen feed and discharge ends. Exhaust volume require-
ments vary with the surface area of the screen and the amount of
open area around the periphery of the enclosure. A well-designed
enclosure will have a space of no more than 2 to 4 in. around the
periphery of the screen.
Storage Bins--
Belt- or chute-to-bin transfer points differ from other
transfer points in that there is no open area downstream of the
transfer point. Hence, emissions are emitted only at the loading
point. The ventilation point is normally located at some point
remote from the loading point and exhausted at a minimum rate of
2
200 cfm/ft of open area. Small fabric filter collectors (2000
to 6000 acfm) are then used to clean the dust-laden exhaust
2-43
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stream. The fabric filter may also be used to collect dust
emitted from loadout operations beneath the storage bins.
Snorkel hoods, which consist of an annular space around the
opening of the discharge chutes, are an effective means of
minimizing particulate emissions that occur as trucks are being
loaded with stone product.
2.2 EVALUATION OF VARIOUS CONTROL ALTERNATIVES
A number of factors that must be weighed in selection of a
control device for a specific process and plant are given in
Table 2-5.
In terms of the technical competitiveness of the various
control devices under consideration, the processes often dictate
the selection of the control device. For example when the revised
kraft pulp mill emission standards are implemented, it may
become necessary to develop hybrid systems either extending the
capability of the existing unit or adding other generic control
devices. Especially with regard to bark boilers, the historical
use of mechanical collectors alone will no longer be adequate to
meet proposed regulations. Increasing efficiency requirements
may cause a shift in the selection of control devices as well as
a move to industrial energy conservation.
2.3 ELECTROSTATIC PRECIPITATORS
2.3.1 System Design Parameters
This section deals with the major parameters that must be
weighed in design of an electrostatic precipitator. The process
applications under consideration are recovery furnaces and
2-44
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Table 2-5. FACTORS BEARING ON CONTROL DEVICE SELECTION
Characteristics of
particles and gas stream
Facilities, costs, legal
factors
Particles
Electrical properties
(precipitators only)
Resistivity
Dielectric constant
Physical properties
Surface properties
abrasiveness
porosity
Density
Shape
Hygroscopic nature
Adhesivity
Cohesivity
Particulate concentration
Size distribution
Gas stream
Flow rate
Process
Viscosity
Chemical composition
Acid constituents
Alkaline constituents
Sulfur oxide content
Moisture content
Plant facility
Waste treatment
Space restriction
Product recovery
Water availability
Co_s_t of control
Engineering studies
Hardware
Auxiliary equipment
Land
Structures
Installation
Start-up
Power
Waste disposal or recycle
Water
Materials
Gas conditioning
Labor
Maintenance
Taxes
Interest on borrowed capital
Depreciation
Insurance
Return on investment
Regulations
Maximum particulate and SO,,
emission rates allowed by
Federal, state, and local laws
2-45
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bark/fossil-fuel boilers in the kraft pulp mill industry. The
dust emitted includes sodium sulfate, sodium carbonate, and
sodium chloride from recovery furnaces and carbon (char), sand,
and fly ash from bark/fossil-fuel boilers.
These industrial applications are essentially all retrofit
pollution control systems (and therefore site-specific). Varia-
tion in processes can cause wide differences in expected per-
formance.
The design procedure is straightforward; given certain
input variables (process application, process flow correlations
and parameters, and applicable particulate emission standard)
and applying experience and theory, one can develop a design
that meets the criteria for efficiency and cost. This format is
illustrated in Table 2-6.
Table 2-6. PARAMETERS AFFECTING PRECIPITATOR DESIGN
System input
Basic design
parameters
Specific design
parameters
System output
Process appli-
cation
Process condi-
tions
Applicable
erni ssion
standard
Total acfm
Total col-
lection area
Power
density
Gas and dust
characteristics
Precipitator
capacity
Electrical/
mechanical
Electrical
energization
System perform-
ance
Overall and
fractional mass
collection
efficiency
Capital invest-
ment
Annual cost
Basic Design Parameters--
The objective here is to determine from the process applica-
tion, process conditions, and applicable emission regulations the
values for gas volumetric throughput (acfm), total plate collection
2-46
-------
2 2
area (ft ), and power density (watts/ft of collecting surface)
These three parameters form the basis for precipitator design.
The total gas volume is dictated by process and production
level. Knowing the total acfm and the specific collection area
2
(SCA, ft /1000 acfm) , one can determine the total area required
to comply with an emission regulation. The equations for doing
so are given below.
SCA = (Eq
wk
i
where
~ = Overall mass collection efficiency, percent
b = Slope of line (reference line slope = 0.5)
w = Modified migration velocity, ft/sec
K.
C = Allowable outlet grain loading, gr/dscf*
C. = Inlet grain loading, gr/dscf*
The proposed Federal standard for new recovery furnaces is
0.10 g/dscm** of saltcake emissions and is equivalent to 0.04
gr/dscf. Based on a typical inlet concentration of 3.8 gr/scf
for a conventional recovery furnace, this corresponds to a
required control efficiency of 98.95 percent. Where codes are
based on pounds of particulate emitted per air-dried ton of pulp,
the 0.04 standard may not be stringent enough for high product
processing levels.
* dscf = dry standard cubic foot.
** dscm = dry standard cubic meter.
2-47
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The presence of fines, although representing a small weight
fraction, may cause a visible plume. Where "equivalent opacity"
is used as a standard, a visible plume may not be acceptable.
The following excerpt from rules and regulations for kraft
pulp mill applications in the Federal Register illustrates
Q
proposed standards.
"The proposed standards would limit emissions of particulate
matter from three affected facilities at kraft pulp mills.
The limits are (1) 0.10 gram per dry standard cubic meter
(g/dscn) for recovery furnaces, (2) 0.15 gram per kilogram
of air-dried pulp (g/Kg ADP) for smelt dissolving tanks, (3)
0.15 g/'dscm for lime kilns when burning natural gas and (4)
0.30 g/dscm for lime kiln when burning oil. Visible emis-
sions from recovery furnaces would be limited to 35 percent
opacity. "
Migration velocity—The modified migration velocity is a
function of electrical energization of the precipitator and of
gas properties. It is often conveniently linked with resistivity
level. A representative range of migration velocity in the pulp
and paper industry is 0.21 to 0.31 ft/sec. Because of the vari-
ation in the ash resistivity due to cyclic operation of the
pulping cycle a precipitator designed for recovery furnaces may
have an uncertainty of approximately 15 to 20 percent in pre-
cipitator migration velocity. For example, in a precipitator
designed for 98 percent collection efficiency, the measured
9
efficiency could vary from 97 to 98.40 percent. A typical
migration velocity range in bark/fossil-fuel boiler applications
is 0.2 to 0.5 ft/sec.
A digression is in order at this point to clarify the usage
of w (modified migration velocity) in contrast to the effective
2-48
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migration velocity w, which is used in the conventional Deutsch-
Anderson efficiency equation. The effective migration velocity,
w, is a function of several factors, including precipitator
length, overall mass collection efficiency, and gas velocity.
The variation in w within a given precipitator is caused by
changing particle size distribution as precipitation proceeds in
the direction of gas flow.
The modified migration velocity, w , as presented by Matts
K
and Ohnfeldt can be treated essentially as a constant for any
application. It is, of course, strongly dependent upon the inlet
particle size distribution.
Power input--The third basic design parameter is power
density required to establish the optimum voltage-current charac-
teristics of the corona, given the dust entering the precipita-
tor. Power density is a function of electrical resistivity,
particle size characteristics and distribution, gas loading and
composition, gas temperature, and gas pressure. It is often
conveniently linked with resistivity, such that for a moderate
g
resistivity of 10 ohm-cm the value will be approximately 2,5
2
watts/ft . For recovery furnaces a typical range of power input
2
is 1.1 to 1.5 watts/ft , as calculated from field operating data.
2
For bark boilers a range of power input is 1 to 3 watts/ft .
Table 2-7 illustrates a general correlation between power
density and dust resistivity, based on typical values for fly
ash.
2-49
-------
Table 2-7. DESIGN POWER DENSITY
Resistivity, ohm-cm
lO4'7
107'8
109'10
1011
io12
>io13
Watts/ft
collecting
4.0
3.0
2.5
2.0
1.5
<1.0
2 of
plate
Operating voltages from field data can range from 45 to 55
kV for 9-inch plate spacing. Current densities, also from field
data, range from 0.02 to 0.05 mA/ft for recovery furnace applica-
tions. Field voltages and current densities for bark boilers
range from 40 to 45 kV and 0.02 to 0.06 mA/ft , respectively.
Thete values are not constant for each point in the precipitator.
At the inlet section where the dust loading is greatest, the
voltage-current characteristics will be significantly different
from those at the outlet.
It appears that resistivity plays a significant role in
selection of w, and power density, yet there is no precise or
}C
universally applied method for predicting resistivity from the
material entering the furnace or process and the process condi-
tions .
Specific Design Parameters--
Table 2-8 is a compilation of design parameters and input
variables grouped in logical categories.
Precipitator Size—One of the first structural parameters
2-50
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Table 2-3. DESIGN PARAMFTERS AND DESIGN CATEGORIES F0"l
ELECTROSTATIC PRECIPITATORS
Dust composition -*•
NaCl
C (Char)
Sand
Fly ash
Precipitator capacity
No. precipitators
No. chambers (units) /precipitator
No. ducts/chamber (unit)
Duct spacing
Plate height
Treatment length
Section lengths and total no. of each (per precipitator)
Collecting area
No. electrical sections parallel to gas flow (per precipita-
tor)
No. electrical sections across gas flow (per precipitator)
No. hoppers parallel to gas flow (per precipitator)
No. hoppers across gas flow (per precipitator)
Rapping, electrodes, etc.
Type discharge electrode
Length discharge electrode/vibrator or rapper
Type discharge electrode/vibrator or rapper
Type collecting electrode
Area collecting electrode/rapper
Type collecting electrode rapper
2-51
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Table 2-8 (continued!
Electrical energization (of each electrical section)
Power density
Length of collecting electrode/T-R
Mode (switching)
Corona power
Current density
Current/T-R set
t_ed_ parameters
Overall mass collection
efficiency
Fractional mass collection
efficiency
Gas flow
Gas teroerature
Gas (treatment) velocity
SCA
Inlet grain loading
Outlet grain loading
2-52
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to be determined is the width of the precipitator (s ) . This
value is dependent on the total number of ducts as determined
from Equation 1.
Total no. ducts = TYrvToTTpTs . ) ( P . H . ) (Eq' 2)
where
acfm = total gas volumetric throughput, acfm
T.V. = gas (treatment) velocity, ft/s
P.S. = plate spacing, ft
P.H. = plate height, ft
A practical approach from the standpoint of energization
and reliability is to limit the total number of ducts per chamber,
The number of chambers is determined by the total number of
ducts, which is determined from Equation 2 and the associated
criteria. The total number of precipitators needed will depend
on the degree of reliability required, space limitations at the
plant site, and the relative ease with which the effluent gas
can be distributed to the precipitator (s ).
The second general design equation provides a guide to the
length of the precipitator, which is dependent on treatment
velocity, plate spacing, plate height, gas volumetric throughput,
and total collecting surface. (Eq. 3)
Treatment _
length
Total collection surface
(No. pptrs.)(No. chambers/pptr.)(No. ducts/chamber)(P.H.)(2)
2-53
-------
The design treatment length will be determined by selection of an
integer value of standard section (field) lengths that may be
offered by the precipitator manufacturer. If it is found, for
example, that four fields are required, two of one length and two
of another, structural considerations such as hopper spans may
determine the positioning of the fields in the direction of gas
flow.
Mechanical sections result from the chamber-wise and field-
wise sectionalization of an electrostatic precipitator. Hopper
selection is based on the size of the mechanical sections and the
predicted inlet grain loading.
Treatment velocity (T.V.) is a function of dust resistivity.
In recovery furnace applications, the precipitator treatment
velocity ranges from 2 to 5 ft/s; in bark/fossil-fuel boiler
applications the range is 3.2 to 4.5 ft/s. The lower velocity
should be considered when burning salt-water-soaked bark material.
In general, the less resistive the dust, the shorter can be the
treatment time.
Plate spacing is more or less fixed by the precipitator
manufacturer, depending on his experience with the various
process applications and conditions, predicted velocity distri-
bution across the precipitator, and type of plate. Plate spacing
usually ranges from 9 to 12 inches.
In selecting plate height, the designer attempts to maintain
both the required treatment velocity and an adequate aspect
2-54
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ratio. The aspect ratio is defined as the ratio of the length of
a precipitator to its height. Historical data indicate that it
can vary from 0.75 to 1.5. The practical limitations on plate
height imposed by structural stability requirements are obvious.
Each manufacturer limits the practical plate heights in accord-
ance with the overall design.
The total number of ducts dictates the width of the pre-
cipitator. The designer next requires some indication of the
system sectionalization, as indicated in Figure 2-10. Chamber-
wise (parallel) sectionalization is sectionalization across the
gas flow, whereas series sectionalization is in the direction of
aas flow.
•H
m
SERIES www
SECTIONALIZATION cQ m «
4
K K S
i U U 0
3rd FIELD
2nd FIELD
1st FIELD
SERIES AND PARALLEL
SECTIONALIZATION
t
PARALLEL
SECTIONALIZATION
t
(GAS FLOW INDICATED BY ARROWS)
Figure 2-10. Sectionalizatior of a precipitator.
2-55
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Discharge/collection electrodes--The geometry of the dis-
charge electrode (fine, barbed, rigid, etc.) will determine
corona current-voltage characteristics. The smaller the wire or
the more pointed its surface, the greater the value of corona
current for a given voltage. Very fine or exotic wires, howevesr,
may have potential for breakage and possible dulling of spiked
points with time.
The maximum value for length of standard 0.109-in.-diameter
discharge wire per vibrator is 3000 ft in recovery furnace
precipitators and 3500 ft in bark boilers. The area of collect-
ing surface per vibrator or rapper usually ranges from 2000 to
2
2500 ft in bark boiler precipitators.
Baffles are used to provide stiffness for support of the
collecting plate and a region of low turbulence to minimize
reentrainment of dust, particularly during rapping. Although a
variety of plates are commercially available, their functional
characteristics are not substantially different.
V i b r a t o r s / r a p p e r s - - Th e air vibrator imparts energy at high
frequency to the discharge electrodes and collecting plates.
The system is designed to create a vibration in the collecting
plates and discharge electrode wires to dislodge accumulations
of particles so that the plates and wires are kept in optimum
operating condition. A pressure setting of 35 to 40 psi on the
pneumatic vibrator is generally adequate. Rappers are occasion-
ally used on kraft pulp mill applications instead of vibrators
for cleaning wires.
2-56
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Rappers can be pneumatic or electromagnetic. Single-impact
(magnetic-impulse, gravity-impact) rappers are often used. The
rapping intensity is determined by the height of the rapper when
released from its elevated position and by the plunger weight.
The weight of the plunger may be from 8 to 32 Ib. The frequency
of rapping is determined empirically by observing the values of
opacity and overall mass collection efficiency measured as the
intensity of rapping is varied.
Electrical energization--The way in which a precipitator is
energized has a great effect on its performance. Selection of
the energization system is probably as site-specific as it is
process-specific. The chief considerations in both utility and
industrial applications are number and size of T-R sets, the
number of electrical sections, degree of high-tension sectionali-
zation, half wave - full wave (HW-FW) operation, and changes in
voltage-current characteristics as precipitation proceeds in the
direction of gas flow.
A mechanical section by definition may become an electrical
section (bus section) if it can be separately energized. Within
an electrical section one may have a chamber-wise or field-wise
high tension split or both (see Figure 2-11). The advantage of
splitting a mechanical section both chamber-wise and field-wise
is increased reliability with, of course, increased cost. For
the applications under consideration, both inlet and outlet
sections are often split in the direction of gas flow. In this
configuration the effects of variations in temperature and dust
2-57
-------
MECHANICAL SECTION MECHANICAL SECTION MECHANICAL SECTION
FIELD-WISE
CHAMBER-WISE
t
[ELECTRICALLY DIVIDED
4 WAYS, PARALLEL AND
SERIES)
CHAMBER-WISE
t
(ELECTRICALLY
DIVIDED 2 WAYS,
IN PARALLEL)
FIELD-WISE
t
(ELECTRICALLY DIVIDED
2 WAYS, IN SERIES)
(DIRECTION OF GAS FLOW INDICATED BY ARROWS)
Figure 2-11. High tension splits.
loading across the chamber because of poor flow characteristics
in ductwork leading to these retrofit units are minimized.
Reliability is increased at the inlet, which is often in the
half-wave mode. If one bus section fails, a jumper cable can be
engaged to apply full wave to the "companion" bus section and
thereby prevent that bus section from failing.
Reliability relates not only to sectionalization of a given
collection area but also to the addition of collection area or
electrical fields. The degree of reliability can be defined in
terms of redundant capacity, which is a function of anticipated
failure and time between maintenance periods. Redundancy may be
defined as that additional area in a precipitator that compensates
for the "normal" level of unavailable collecting area. To
2-58
-------
achieve a reliable yet cost-competitive design, one must have
detailed information on dust characteristics and a clear under-
standing of the effect of process variations on precipitator
performance.
The basic consideration in energizing the precipitator is to
maximize the power input to achieve the highest collection
efficiency from a given collection area. The decision on the
degree of sectionalization, however, is made quite independently
of the way in which the precipitator is to be energized. The
number, size, and mode of operation (HW or FW) of the T-R sets
can be manipulated to provide the required current density
within each bus section of the precipitator. The size of the
transformer rectifer (TR) sets is selected to provide lower
current density at the inlet.
In spark-limited operation, half-wave allows time during the
off-half cycle for recovery from the sparking condition (spark
quenching). Complete decay of the charging field and of collec-
tion efficiency during the off-half cycle is avoided because of
the capacitive effect of high-resistivity dust, which tends to
maintain the field potential.
In cases where the resistivity of the dust has been reduced,
i.e., because of in creased temperature of operation or high
carryover of carbon the capacitative effect of the dust is also
reduced. Thus, the charging field decays move in half wave then
full wave.
2-59
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The selection as to the mode of operation is site-specific,
and the variability of performance measurements in full-scale
precipitators may overwhelm any differences due to operation in
either mode. In the processes under consideration, half-wave
has been often used in the inlet fields and full-wave in the
outlet fields.
In summary, precipitator sectionalization and energization
are based on maximizing the power input to the precipitator to
achieve the highest efficiency from a given collection area
while minimizing the potential for poor performance as a result
of various failure patterns. The reliability of precipitator
performance is a function of process flow conditions and dust
characteristics, reliability requirements, and the designer's
experience. The way in which a precipitator is energized depends
on the sectionalization configuration and the current density to
be supplied to each bus section, as determined by chemical and
physical characteristics of the dust, dust loading, and the gas
stream. The number, size, and mode of operation of the T-R sets
can be fitted to the sectionalized configuration after bus sec-
tion design has been established.
2.3.2 Correlations^
The foregoing discussion of precipitator design shows that
three parameters are of central interest: gas volumetric through-
put (acfm); total collecting area (ft ); and power density
(watts/ft of collecting surface). The graphical correlations
discussed in this section relate these basic design variables to
2-60
-------
process application. The designer's judgment, experience, and
understanding of precipitator theory allow him to select the
values of overall mass collection efficiency, SCA, and power
density required for given process conditions and emission
standards. A word of caution is needed, however. It is not
intended that the approach presented here should be directly
applied to a specific site or installation. In such applications
a number of very practical points must be considered, such as
design features to control and minimize large-scale turbulence,
gas sneakage, and particle reentrainment in the precipitator.
The following discussion will examine some design trends
that have been established for precipitator sizing in the kraft
pulp mill industry. The following processes are discussed:
1) Conventional recovery furnaces
2) Low-odor recovery furnaces
3) Bark/fossil-fuel boilers.
ESP Design Correlations - Conventional Recovery Furnaces--
The basic pulping process is described in Section 2.1.1. In
this section the effect of the various process parameters affect-
ing the performance of the electrostatic precipitators is examined
more closely. For this purpose the important process parameters
are summarized in Table 2-9.
2-61
-------
Table 2-9. TYPICAL OPERATING CONDITIONS FOR
PRECIPITATORS ON CONVENTIONAL RECOVERY FURNACES
Particle size distribution
Grain loading
Particle material Sp. Gr.
(75% Na2S04)
Gas temperature
Particle resistivity
.Moisture content
x- = 1.4 •* 1.9 urn
c = 3.0 + 2.04
3-5 (gr/acf)
2 - 2.5
280 - 325 °F
10 - 10 ohm-cm
20 - 30% by volume
Modified Deutsch >lodel--Matts and Ohnfeldt proposed an
empirical modification of the classical Deutsch equation that
essentially removes the size dependence from w, the particle
_ -^
migration velocity. Their equation is: n = 1 - exp (-w A/Q)
j\.
where k is said to be equal to 0.5 in most cases. It can be
shown, however, that w is dependent on the inlet particle size
JC
14
distribution, which changes with each application even if all
other characteristics of the precipitator are the same.
14
Feldman proposed another efficiency equation of the form
n- = 1-C
(w'A/Q)
m
(Eq. 4)
Equation 4 has an advantage of separating all the effects of size
distribution into the constants C and m and of defining w1, which
is dependent only on the electrical energization of the precip-
itator and gas properties. It should be noted that equation 4 is
still based on the Deutsch model and is subject to its assump-
tions and limitations. The advantages over the Deutsch equation
2-62
-------
are that the "effective migration velocity" is replaced by a
quantity w1 which depends on the electrical and gas properties
and that the two constants C and m in equation 4 account for the
size distribution effects. In the absence of abundant design
data, this equation was used to generate theoretical design lines
for precipitators on kraft recovery furnaces and bark/fossil fuel
fired boilers. This equation has been confirmed in part by
available test data. The derivation of equation 4 is discussed
briefly in the following paragraphs.
The electrical force on a charged particle in an electric
field is given by:
F = qE (Eq. 5)
where E (from the Deutsch model) is the electric field strength
at the precipitator collecting electrode. (See Table 2-10.
Units are in the inks system.) The force opposing particle
motion through the gas is:
F = STTP dwd/CI (Eq. 6)
Equating forces and solving for the migration velocity of parti-
cles of size d:
"d = 3^f (E^ 7)
CI is the Cunningham correction factor given by:
CI = 1 + 2.5A/d + 0.84A/d exp (-.435/A) (Eq. 8)
The particle charge q can be represented by the Cochet equation:
2-63
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Table 2-10. NOMENCLATURE FOR THE
ELECTROSTATIC PRECIPITATOR COMPUTER MODEL
A = precipitator collecting area
a = particle-size-dependent parameter, Eq. (11)
C = constant defined in Eq. (19)
CI = Cunningham's slip correction, Eq. (8)
d = particle diamter
E = peak charging field
E = electric field near the collecting electrode
P
F = Stoke's drag force on the particle
f, (d) = inlet particle size distribution function
g(d) = particle size dependent parameter, Eq. (14)
Kr = particle relative dielectric constant
k = constant defined in Eq. (13)
m = constant defined in Eq. (19)
n = number of mechanical sections
0 = volumetric gas flow rate
q = particle charge magnitude
r = particle fraction assumed reentrained at every
mechanical section
SCA = specfic collection area
SCAr = specific collection area of precipitator on
conventional recovery furnace
SCAT = specific collection area of precipitator on low odor
recovery furnace
w = migration velocity
w, = migration velocity of a particle of size d
w, = overall effective migration velocity
/C
[continued) 2-64
-------
Table 2-10 (continued]
w1 = modified migration velocity defined in Eq. (22)
w' = modified migration velocity on conventional recovery
furnace application
w' = modified migration velocity on low odor recovery
furnace application
x = mass mean diameter
E = permittivity of free space
'-., = collection efficiency of particle size d also
efficiency correct for reentrainment loss r on
particle size d
"i* = collection efficiency on particle size d without
reentrainment
~ = overall collection efficiency of size d, EC. (35)
>' = mean free path of gas molecules
'»' = empirical mean free path in Eq. (9)
i. = dynamic gas viscosity
c = geometric standard deviation of the particle size
distribution
F. = product
2-65
-------
where
Kr = relative dielectric constant of the particle
and
X1 = empirical mean free path
= 0.1 ^m at 20°C and 1 a tin. pressure
The Cochet equation accounts for particle charging by both
field charging and diffusion charging mechanisms. This is
important in analyzing the effects of particle size, since the
charging mechanism changes from field charging to diffusion
charging in the submicron range. Although there are more accur-
ate methods of computing particle charge than the Cochet equation,
these require numerical solution. For the purposes of this
discussion, the Cochet equation is entirely adequate in repre-
senting the effect of particle size on charge.
Combining equations 7 and 9 and defining
* = [(1 + ^cf)2 * II + 2X-/d) (rV2^' (E^ 10>
the particle migration velocity for particles of size d becomes:
GEE
w = ( G ° P) aCId (Eq. 11)
d j u
For particles of a single size, d, the Deutsch equation can
be applied to calculate efficiency:
1 - n ) = exp[-WdA] = exp [-(CoEoEpA) aCId] (Eq. 12)
Q 3,Q
Defining new terms:
k = £oEoEpA (Eq. 13)
3uQ
g (d) 5 aCId (Eq. 14)
2-66
-------
The single-size efficiency equation becomes:
(1 - r,d) - exp[-kg(d)] (Eq. 15)
The overall collection efficiency is found by integrating over
the inlet size distribution, f (d):
(!-")= o/°° (I - nd) fI (d)dd (Eq. 16)
Assuming a log-nornal inlet distribution this becomes:
- 1 TOO
~ M = vv^ ^ c/ exp[-kg(d)-0.5(r]dlnd(Eq. 17)
If the integral in this equation could be evaluated directly,
one would have the overall efficiency equation in terms of x, c,
and k; i.e. size-dependent variables would be distinguished from
the other variables. However, the integral must be evaluated
numerically, and therefore no simple analytic expression for
efficiency is obtained directly. It is possible to arrive
indirectly at a simple efficiency equation through use of equa-
tion 17. The procedure is as follows.
Equation 17 is solved numerically on the computer to obtain
different r. , k pairs for the typical inlet particle size distri-
bution in a recovery furnace, d ^ 1.7 urn, and x = 2.5. The
results are plotted on log-log paper for [-In (1-n) ] versus k.
As shown in Figure 2-12, this plotting yields a straight line,
which indicates a direct relationship between (1-n) and k. The
equation of a straight line on this plot can be written as:
In
= m ln(k/k ) (Eq. 18)
2-67
-------
100
c
i
1.0
0.1
J L
_UL
10"
10'
Figure 2-12.
Plot of k versus -ln(l-n) for recovery
furnace applications.
2-68
-------
- 4
If C is defined as a value of (1-n ) when k = 10 , equation (1!
becomes:
In
InC
or
Ind-nl
= m In (10 4k) (Eq. 19]
M n~i- "i
(!_-) = CUU K) (Eq. 20)
-4
By referring to equation (13), the term 10 k can be written as:
10~4k = w1 | (Eq. 21)
where
L E E _4
w1 = —" ^ P x 10 (Eq. 22)
J H
Thus, the desired efficiency equation is:
1 - - = C(W'A/Q)m (Eq. 23)
where w' is a modified migration velocity (its dimensions are
sec ) and is independent of the inlet particle size distribution
but is completely dependent on the electrical energization of the
precipitator.
Model application to conventional kraft pulp mills--To
generate absolute numbers for overall efficiency and SCA, w1 must
be estimated. Available electrical data from field precipitators
(see Table 2-11) can be used to determine w', as follows:
w. ; £°^EP x lO'4 (sec'1)
where E is the peak charging field and E is the average elec-
tric field near the collecting plate commonly known as the
collecting field. Estimates of E could be made knowing the
secondary voltage wave form to the precipitator and the absolute
2-69
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magnitude of the average secondary voltage at the discharge
electrode. Generally full-wave rectifiers are used on kraft pulp
mill precipitators and the secondary average operating voltage is
on the order of 48 kv. This results in
48 x 103 x 2
0 ' 6.1143 (Eq. 24)
EO = 5.95 x 105 (v/m) (Eq. 25)
Table 2-11. TYPICAL ELECTRICAL OPERATING DATA ON STANDARD
9-INCH PLATE PRECIPITATOR WITH 0.109-INCH DISCHARGE ELECTRODES
Avg. secondary voltage at the
discharge electrode
Avg. plate current density
Avg. power density
45-55, kV avg
10-50, mA/1000 ft
2
1-2.5, W/ft
2
Analytical expressions to estimate the collecting electric
field are relatively hard to derive in the case of wire-plate
precipitators. A simple expression for E can be derived for
P
present purposes, assuming low current density. ' Available
field data on a 9-in. plate precipitator with 0.109-in. diameter
discharge electrodes indicates that for this application, V is
on the order of 23 kV average at 300°F. For a typical operating
voltage of 48 kV average, it is estimated that E will be on the
order of 4.5 x 10 (V/m). The dynamic gas viscosity (•_) at 300°F
is theoretically estimated at
p - 2.5 x 10~5 (Nt-s/m2) (Eq. 26)
Thus, substituting the various quantities in equation 22 one
obtains:
w. = L15i_x_l_OlJ:JJL.5._95-X J^jjc.1^ x 105 x 1Q-4
3 x 2.5 x ..0
2-70
-------
w' = 3.155 (s ) (Eq. 28)
Therefore, for the recovery furnaces on kraft pulp mills, the
design equation becomes:
. -, /-i — » 1/m
A X ln d-n)
Q ~ 3.155 InC
The inlet particle-size-dependent parameters C and m are deter-
mined by solving equation 17 numerically on the computer for the
typical inlet particle size distribution of d = 1.7 ym and c =
2.5. The values of C and m are:
C = 0. 957; m = 0.89
This design equation for precipitator size on the conventional
kraft pulp process mill is given by:
A 1 IrWT n> 1/0.89
A _ In(l-n)
__
Q ~ 3.155 ln(0. 957)
To convert from mks to English units the value for A/Q (sec/m) in
equation 30 should be divided by 0.197, thereby obtaining ft /1000
acfm.
Figure 2-13 shows the design line predicted using equation
30 and the test data points from the various jobs on conventional
recovery furnaces. Correlation between the predicted model
efficiency and the performance test data is excellent.
Precipitator Design Correlations for Low-odor Recovery Furnaces —
Efforts of the pulp and paper industry to eliminate odorous
gases escaping from pulp mills often involve switching to the so-
called low-odor recovery process. In this process the odor
levels are lower, but the particulates emitted have different
2-71
-------
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2-72
-------
physical properties. R.L. Bump has summarized some important
differences that must be accounted for in sizing a precipitator
for a low-odor recovery furnace. The differences affecting
precipitator performance are summarized in Table 2-12.
Table 2-12. DIFFERENCES IN PARTICULATE PROPERTIES
IN CONVENTIONAL AND LOW-ODOR RECOVERY PROCESSES
Parameter
Temperature, °F
Moisture content, %
Bulk density, Ib/ft^
Tenacity
Sulfur content
Conventional
280-325
20-30
20-25
Moderate
Low
Low-odor
340-450
7-20
5-10
High
High
The dynamic gas viscosity plays an important role in deter-
19
mining particle drag force. Typical compositions of flue gas
entering the precipitator on a conventional recovery boiler '(wet
basis) and a low-odor recovery boiler are shown in Table 2-13.
Assuming typical gas temperatures of 300°F and 400°F in conven-
tional and low-odor applications and assuming 1 atmosphere
absolute pressure, one can calculate the dynamic gas viscosities
for the two applications. Calculation indicates that
p low-odor
conventional
= 1.15
(Eq. 31}
2-73
-------
Table 2-13. TYPICAL ANALYSIS OF FLUE GAS FROM
CONVENTIONAL AND LOW-ODOR RECOVERY BOILERS19
Component, %
N2
co2
CO
°2
Temperature, °F
Dynamic viscosity,
Nt-s/m2 x 10~5
Conventional
62.3
12.6
0.08
1.93
23.10
300
2.07
Low-odor
73.64
14.9
0.09
2.27
9.1
400
2.39
Other important parameters in precipitator sizing are particle
size distribution, particle resistivity, the electrical current-
voltage data, and particle reentrainment. Available particle
size data on the low-odor process indicates mass mean diameters
of about 1.5 ym and a standard deviation of about 2.5. These
values are very similar to those of particulate from the conven-
tional recovery furnace. Particle resistivity in the two pro-
9
cesses is the same order of magnitude, around 10 ohm-cm. The
electrical data could be different with lower operating voltages
on high-temperature, low-odor processes. The peak charging
fields would be expected to be lower. This effect would be
compensated for, however, by an increase in ion mobility and
higher precipitator current. Preliminary calculations indicate
that the available electrical data do not explain the differences
in precipitator performance caused by greater temperature
fluctuations.
2-74
-------
The tenacity of the dust and the bulk dust density are very
important in quantifying rapping reentrainment losses. Although
only qualitative statements can be made in the absence of quan-
titative measurements, field experience indicates that in the
low-odor process the dust adheres very tenaciously to the collect-
ing plates. To dislodge this dust sufficient rapping forces must
be applied to produce rapid acceleration parallel to the gas flow
(shear action). With heavy rapping, vibrations can be induced
perpendicularly to the gas flow direction in addition to the
necessary shear action at the plate surface, which result in a
scattering of the agglomerate. This can lead to reentrainment of
relatively larger fractions of the collected particulate in a
low-odor process than in the conventional processes.
Dynamic viscosity of the flue gas determines the drag force
on the particles. The higher the temperature, the higher will be
the viscosity, resulting in increased drag force on the particles.
As shown in equation 29, SCA is inversely proportional to w1, the
modified migration velocity, according to the precipitator model
presented in connection with the conventional recovery furnace
process. Assuming that all things are equal in the two processes
except the gas dynamic viscosity, one can write
SCA w'
SCA~ = wf = X-15 (E^ 32)
C -LJ
Equation 32 illustrates that a precipitator applied to low-odor
process (operating at 400°F) requires an SCA about 15 percent
higher than one on a conventional process (operating at 300°F.)
2-75
-------
A precipitator design line based on these calculations is shown
in Figure 2-14 for precipitator sizing on low-odor recovery
furnaces. Additional test data are required to determine the
validity of equation 32.
Precipitator Design Correlations for Bark. Boilers in the Paper
Industry--
The objective here is to outline the design method used to
size precipitators applied to collection of bark ash emitted from
power boilers. This evaluation does not include emissions from
boilers fired with sea-soaked bark materials. It is well known
that the low resistivity of bark ash can pose serious collection
problems. Reentrainment losses are high. Nevertheless, ESP's
have been used successfully on non-salt laden bark boilers. Any
theoretical model would overpredict precipitator performance
unless the calculated efficiencies are corrected for rapping
reentrainment losses as is done in the model presented here.
Details of the bark boiler combustion process are described
in Section 2.1.1. For theoretical development purposes the key
parameters are summarized in Table 2-14.
2-76
-------
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2-77
-------
Table 2-14. TYPICAL OPERATING CONDITIONS IN PRECIPITATORS
ON BARK/FOSSIL-FUEL BOILERS13
Particle size distribution:
Limited data on particle size analysis indicate the
following ranges
x = 5 - 15 (ym)
o = 2.5 - 4
Grain loading: 0.5 - 1.5 gr/acf
Particle material sp. gr.: 0.5 - 1.0
Gas terperature: 300 - 400 °F
Particle resistivity: 10"^ - 10^ ohm-cm
Moisture content: 10 - 20% by volume
Typical Electrical _0_p_er_a_tJ.jTg_ Data (average values)
Secondary voltage at discharge electrode: 40 - 45
(kV avg.) electrode 2
Plate current density: 20 - 60 mA/1000 ft
Power density: 1.5 - 3 W/ft2
Theoretical models—The Deutsch model used earlier in connec-
tion with recovery furnaces can be used as a basic equation with
no reentrainment. The Deutsch equation for a single size par-
ticle is given by
"ij = efficiency without reentrainment
= 1 - exp (-w A/Q) (Eq. 33)
Letting r be the fraction of collected material reentrained in
the gas stream as a rapping reentrainment loss, one can write
nd = d-r)^ (E3- 34)
where
n, = efficiency corrected for the reentrainment loss r.
Therefore, for the entire precipitator length one can write
1 - nT = n(l-nd) = n{l-(l-r)nd} (Eq. 35)
2-78
-------
where
r] = overall collection efficiency of size d
Equation 35 can be further simplified if the following assumptions
are made: 1) overall migration velocity remains constant; 2) the
fraction of the material reentrained remains constant for differ-
ent particle sizes; and 3) that the fraction of material reen-
trained remains constant for each mechanical section.
With these assumptions equation 35 can be written to obtain:
1 - - = -1 - d-r) (1 - exp(-wdA/0) ) -n
or
1 - r = -r + (l-r)exp(-wdA/Q)}n (Eq. 36)
where
n = number of mechanical sections in the precipitator.
The overall mass collection efficiency of the precipitator
can then be determined by integrating equation 36 over the
entire particle size distribution. Therefore, mathematically one
can write
d-7) = i° (1--,T) f1(d)dd (Eq. 37)
o
Assuming a lognormal inlet particle size distribution, equation
37 can be simplified. The final integration, however, requires
a numerical technique to establish the relationship between r on
^i.
Model application to bark boilers--The method used here is
similar to that described above for recovery furnaces.
Eo = ^TTll43 X 2 = 5.07 x 105 V/m (Eq. 38)
2-79
-------
Using the computer program and Cooperman's correlation for the
current-voltage information, we estimate
E = 3.7 x 10 V/m
P
y = 2.5 x 10~5 Nt-s/m2
£ = 8.854 x 10~ coul/V-m
Q
w1 = 2.216 s~l
Therefore, for bark ash boilers the theoretical design equation
assuring no reentrainment becomes
1/m
A = f_±_^> An(l-~ A (Ec. 39)
Q \2. 216^ ^ InC J
The values of m and C can be determined as explained in the
previous section. Assuming x = 5 urn and c = 2.5 the calculated
values are
C = 0.7166; m = 0.63
To convert from mks to English units, the value for A/Q in
sec/m in equation 39 is divided by 0.197, thereby obtaining
ft2/1000 acfm.
The plot of SCA versus overall efficiency for bark/fossil
fuel-fired boilers is shown in Figure 2-15. The efficiency
values with no reentrainment (r=o) over-predict precipitator
performance and must be corrected for reentrainment losses. Use
of the reentrainment model presented in this section (at r=40 and
50%) shows a closer correlation to available test data on the
two installations shown in Figure 2-15. Field data generally
show a 40 to 50 percent loss in efficiency due to reentrainment.
The model needs further testing with additional performance
test data before it can be widely applied as a design tool for
2-80
-------
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4J w
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r—I QJ
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l-i -H
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en
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r I
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t-i C
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2-81
-------
sizing precipitators applied to bark boilers. In Figure 2-15,
use of the model assumes a typical inlet distribution of x = 5
and a = 2.5. The reference line has a slope of 0.5.
2.3.3 CajDiJt a_l_ a n d _Ann u a_l_ C o sts__o f E_l_e ctrost atic P r e cipitators
on Kraft Pulp _Mill ^Applications
Capital and annual cost correlations for electrostatic pre-
cipitators on kraft pulp mills were developed using predicted
SCA's from Figures 2-13 through 2-15, corresponding to conven-
tional (high-odor) recovery furnaces, low-odor recovery furnaces,
and bark/fossil-fuel boilers, respectively. In addition to
standard Research-Cottrell sizing and costing criteria, computer
programs were used to develop the cost correlations. The costs
are presented in Figures A. 2-1 to A. 2-6, in Appendix A-2.
Precipitator capital costs to the user, normalized at year-
end 1977, include the flange-to-flange precipi tator , structural
support, erection and installation, engineering, contingencies,
and warranty and acceptance. Heat jackets are not included for
recovery furnaces or bark/fossil-fuel precipitators.
Annual costs consist of maintenance, labor, power consump-
tion (assuming $0.03/kWh), administration, overhead, and capital
charges (taken at 15% of total capital investment) . Depreciation
(assuming a 15-year accounting equipment life), interest, insur-
ance, and taxes constitute capital charges.
Since capital investments for the low-odor furnace at. 99.8
percent (Figure A. 2-3) are approximately the same as those for
the conventional furnace at 99.9 percent, it is clear that the
2-82
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low-odor option is more expensive. The difference in efficien-
cies alone accounts for a 20 percent larger precipitator size
(SCA) for low-odor applications. Furthermore, the drag-bottom
configuration associated with low-odor applications is more
expensive than the wet-bottom used in conventional recovery
furnace precipitators.
For the efficiency ranges considered, it appears that the
cost of a heat jacket may add up to 5 percent to the cost of the
precipitator system as defined above. Use of a heat jacket
depends on precipitator capacity, shell surface area, and operat-
ing and ambient temperatures.
Annual costs are higher in low-odor (Figure A.2-4) than in
conventional (Figure A.2-2) applications. Since capital invest-
ments are higher for low-odor precipitators, the capital charges
are correspondingly higher. Furthermore, low-odor precipitators
have higher power density (1.9 W/ft versus 1.5 W/ft for con-
ventional precipitators) and hence direct operating costs are
higher.
2.4 MECHANICAL COLLECTORS
2.4.1 General System Characteristics
This section describes the major parameters that must be
considered in the design of a mechanical collector servicing
bark/fossil-fuel boilers. The most common type of mechanical
collector is the conventional cyclone.
2.4.2 Design Philosophy
21
Parameters affecting the design of a cyclone are presented
in Table 2-15, and described in the text that follows:
2-83
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TABLE 2-15. PARAMETERS AFFECTING CYCLONE DESIGN21
System input
System
parameters
System
output
Process application
Process conditions
Applicable emission
standard
Pressure drop
Particle size
distribution
Inlet gas
velocity
Cyclone body
diameter dimen-
sion ratios
Specific gravity
of dust
Overall collection
efficiency
Capital investment
Annual cost
Pressure Drop--
Pressure drop is one of the most important factors affecting
efficiency and design. The pressure drop across a cyclone varies
as the square of the gas volume and is directly proportional to
the density of the dust-laden gas. The total pressure drop in a
cyclone is the sum of separate losses in the inlet flue, the
cyclone body, and the outlet duct.
Particle Size--
The efficiency of a conventional cyclone decreases with
decreasing particle size to a point where particle collection is
50 percent at a particle diameter of 5 um. The finer particles
are strongly influenced by turbulence of gas flow and are not
collected. The effect of particle size on efficiency of a specif-
ic type of cyclone is shown in a fractional efficiency curve,
which can be obtained only from test data (See Figure 2-16).
2-84
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AIR AT 70°F
RESISTANCE 3.0 in. WATER GAUGE
LOAD 46 g/ft3
sp. gr. 2.1
I
10 15 20 25 30
PARTICLE DIAMETER, ym
35
40
22
Figure 2-16. Typical fractional efficiency curve.
Cyclone Body and Diameter Dimension Ratios—
A cyclone of relatively higher efficiency and higher pres-
sure drop could be designed by 1) increasing the length of the
cyclone, 2) decreasing the width of the inlet, or 3) increasing
the ratio of body diameter to outlet diameter, while at the same
time reducing the body diameter. Increasing the length of the
cyclone body provides a longer residence time for gas in the
cyclone and therefore more revolutions of the gas stream. In-
creasing body length also minimizes efficiency loss due to reen-
trainment of particles in the ascending vortex. Increasing the
ratio of body diameter to the gas outlet diameter effects an in-
crease in efficiency at ratios up to about 3 to 4; above a ratio
of 4, the gains are slight but the pressure drop increases.
Specific Gravity of the Dust--
Of the dust physical properties affecting the collection
2-85
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efficiency of a cyclone, specific gravity is one of the most
significant. Efficiency is greater with particles of higher
density than with those of lower density. It is the specific
gravity of the particle, and not the bulk density of the dust
that is important. However, bulk density of the dust is an
important design consideration.
23
Tables 2-16 and 2-17 summarize the effects of cyclone
design parameters.
2.5 WET SCRUBBERS
2.5.1 Gen era 1 __Sy_s_t_e_m_ Characteristics
The sources of particulate emissions to which wet scrubbers
are applied are smelt dissolving tanks, sludge lime kilns, and
more recently combination bark/fossil-fuel boilers at kraft pulp
mills, and conveyors and crushers in stone crushing operations.
Types of wet scrubbers used are impingement, packed towers,
showered mesh mist eliminators, and Venturis.
Exhaust gases from a lime kiln normally pass through a mech-
anical cyclone collector for lime dust recovery and finally
through a liquid scrubber for particulate control. The major
types of scrubbers used on lime kilns are impingement and venturi
scrubbers. Impingement scrubbers have been used extensively for
particulate scrubbing on lime kilns because their low pressure
drops and low scrubber shower rates minimize operating costs. A
disadvantage of the impingement scrubber is a limit on the solids
concentration in the scrubber slurry because of plugging potential.
They are also less efficient than other scrubbers in removing
particulate matter because the gas-liquid contact is less efficient.
2-86
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Table 2-16. PERFORMANCE TRENDS BASED ON CHANGES IN
CYCLONE DESIGN
Change
Increase cyclone size
Lengthen cylinder
Increase ir. " et area,
maintain volume
Increase inlet area,
maintain velocity
Lengthen cone
Increase size of cone
opening
Decrease size of cone
opening
Lengthen clean-gas out-
let pipe internally
Increase clean-gas out-
let pipe diameter
Performance trend
Pressure loss
Down
Slightly lower
Down
Up
Slightly lower
Slightly lower
Slightly higher
Up
Down
Efficiency
Down
Up
Down
Down
Up
Up or down
Up or down
Up and/or
down
Down
Cost
trend
Up
Up
No effect
Down
Up
No effect
No effect
Up
Up
2-87
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Table 2-17. EFFECTS OF PHYSICAL PROPERTIES AND PROCESS
VARIABLES ON EFFICIENCY
Proportional
change
Gas change
Increase velocity
Increase density
Increase viscosity
Increase temperature
(maintain velocity)
Dust Change
Increase soecific
gravity
Increase particle
size
Increase loadings
Pressure
loss
Up
Up
Negligible
Down
No effect
No effect
No effect
Efficiency
Up
Negl igible
change
Down
Down
Up
Up
Up
Cost
trend
Initial cost down
Operating cost up
Slightly higher
No effect
No effect
No effect
No effect
No effect
2-88
-------
Capital costs of impingement scrubbers are higher than costs of
venturi scrubbers on similar installations because the impinge-
ment scrubbers are larger and more complex. Newer kraft pulp
mills are installing venturi scrubbers on the lime kilns because
of their higher particulate removal efficiencies. Venturi scrubbers
can operate with slurry water solids concentrations of up to 30
percent by weight without excessive plugging. Some operating
characteristics of particulate scrubbers on kraft lime kilns are
24
presented in Table 2-18.
Table 2-18. OPERATING CHARACTERISTICS OF PARTICULATE
LIQUID SCRUBBERS ON KRAFT LIME KILNS
Parameter
Liquid/gas ratio,
(gal/1000 ft3)
Slurry solids, %
Pressure drop, mm
(in. H20)
Power required,
(hp/ton/day )
Power required,
(hp/ton/day)
liter/in
by wt
Hg
kW per ton/day
kW per ton/day
Scrubber type
Impingement
0.54-2.0
(4-15)
1-2
9-13
(5-7)
0.041-0.049
(0.05-0.06)
(0.13-0.16
(0.16-0.20)
Venturi
1.73-3.21
(13-24)
10-30
19-28
(10-15)
0.082-0.099
(0.10-0.12)
0.27-0.34
(0. 33-0.42)
per mass of pulp
per mass of lime
The particulate matter from kraft smelt dissolving tanks
consists of both dissolved and undissolved NaOH, Na_CO_., and
Na
_S. Typical particulate emissions from smelt dissolving tanks
are 0.03 to 1.2 kg per Mg of pulp (0.05 to 2.3 Ib/ton) following
control devices.
25
Most of the smelt tanks that are controlled
2-89
-------
use mesh demisters and/or packed towers to collect particulate
matter from exhaust gases.
Typical operating conditions for two recently installed ven-
turi scrubbers on combination bark/fossil-fuel boilers are
summarized below:
Pressure drop,
Volume acfm in. W.G. L/G (gal/1000 ft3
74,575 at 390°F 8-10 6.7 - 8.0
174,000 at 395°F 15-20 8.5
The following are operating conditions specified for venturi
scrubber operations on crushers:
Pressure drop, in. W.G. L/G gal/1000 ft3
At 20,000 acfm 7-17 9-13
At 70,000 acfm 9-16 9-13
The ranges of pressure drops and L/G ratios are attributed to
different levels of scrubber efficiency.
The following are operating conditions specified for venturi
scrubbers at conveyor transfer points:
Pressure drop,
in. W.G. L/G gal/1000 ft3
At 5,000 acfm 7-12 8-9
At 15,000 acfm 7-12 8-9
Impingement Scrubbers--
In an impingement scrubber the gas stream enters the collector,
then passes over a pool of water and impacts the surface of the
pool before exiting. Control of the water level is critical to
this device. Pressure loss and efficiency are determined by the
liquid level. This type of collector functions well in the pres-
sure drop range of 2 to 6 in. Inlet loadings range from 3 to
2-90
-------
14 gr/acf, and outlet emissions range from 0.4 to 1.5 gr/acf.
Although it is advantageous to attempt sludge separation inside
the scrubber, the dust tends to stay in suspension in the sump
because of air turbulence. Nevertheless, where particles are
large and settle easily, this can be a very successful arrange-
ment. Because the liquid in the pool serves as the scrubbing
medium, the liquid discharge rate can be adjusted according to
the desired solids content in both the retained liquid pool and
the discharge sidestream.
Packed Towers--
Packed vertical towers contain materials on which the gas
and liquids are contacted. Raschig rings are the most common
packing materials. The gas stream can be introduced across, with,
or against the flow of liquid. In the crossflow design, liquid
is introduced at the top of the packing, while the gas moves
horizontally through it. This arrangement helps to wash any accu-
mulated contaminants off the packing surface. In the cocurrent
flow design, gas and liquid enter at the top of the scrubber
and leave at the bottom. The exit gas stream then passes through
an entrainment separator or mist eliminator. In the countercur-
rent design, the liquid flows down through the bed, wetting the
packing to provide interfacial surface area for mass transfer of
the pollutant with the gas. The gas then flows up the bed counter-
current to the liquid. Highest pressure drop and lowest gas flow
are achieved with the countercurrent unit; gas flow capacity is
greatest in the cocurrent unit, and operation of the crossflow
2-91
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tower is unpredictable. All three designs are subject to partic-
ulate buildup on the walls. Clean liquid and low dust loading
are desirable. Water is introduced above the packing by weirs or
spray nozzles. Packed towers must be operated within a narrow
range of conditions to prevent maintenance problems. If either
the liquid or gas rate is accelerated, liquid holdup occurs and
pressure drop increases. If gas velocity is further accelerated,
flooding occurs, accompanied by high pressure drop and entrain-
r.ent of liquid in the gas stream. Buildup of solids in the pack-
ing is a serious problem. Unlike other scrubbers, packed tower
2 8
internals are not easily accessible.
Showered Mesh Mist Eliminators--
Particulate matter consisting of dissolved and undissolved
NaOH, Na CO , and Na2S is emitted from the smelt tank with the
flow of gases. The mist eliminator consists of fine wire mesh
screens approximately 30 cm (1 ft) thick. Droplets condense from
the gas on the wire mesh screens, which may be placed in series
as determined by the absorption efficiency required.
Collection efficiency can be increased by following the mesh
mist eliminator with a packed tower or by using a packed tower
only. Table 2-19 summarizes the efficiency of mesh demisters
used in conjunction with another wet scrubber on smelt dissol-
29
ving tanks.
2-92
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Table 2-19. PERFORMANCE CHARACTERISTICS OF SHOWERED
MIST ELIMINATORS ON SMELT DISSOLVING TANKS29
Control device
Pad entrainment
Separator
Separator
Separator
Separator
Separator
Pad plus shower scrubber
Pad plus packed scrubber
Packed scrubber
Collection
efficiency, %
71.8
77.2
77.8
90.2
93.4
70.8
96.2
91.9
98.1
Emission rate,
Ib/ton
(0.05)
(0.15)
(0.63)
(2.3)
(1.2)
(1.58)
(0.41)
(1.20)
(0.05)
Venturi Scrubbers—
In conventional terminology, the venturi scrubber is cate-
gorized as a gas atomized spray scrubber. The collection process
mainly relies upon acceleration of the gas stream to provide im-
paction and intimate contact between the particulates and fine
liquid droplets generated as a result of gas atomization. Basi-
cally, this is a high energy consuming device designed for high
particulate collection e-fficiency. Typically, the pressure drop
is on the order of 10 to 20 in. of water or more in kraft pulp
mill applications. Collection efficiency increases with the pres-
sure drop and liquid-to-gas circulation ratio. However, there is
an optimum L/G ratio above which additional liquid is not effec-
tive at a given pressure drop. In this device the pressure drop
can be increased by increasing the gas velocity.
The system characteristics and design philosophy for wet
2-93
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scrubbers applied to stone crushing and conveyor transfer points
are similar in many respects to the information presented for
kraft pulp mill applications. One difference is that hoods and
sometimes extensive ductwork are required to collect and transfer
emissions from primary, secondary, and tertiary crushers to the
scrubber inlet. The system consists of a venturi scrubber, re-
circulating tank, pumps, slurry settler, slurry filter, and
induced-draft fan. A system for conveyor transfer points requires
less ductwork than one for crushing operations.
2.5.2 Design Philosophy
Where local emission regulations are strict regarding kraft
pulp mill and stone crushing operations, venturi scrubbers are
often used when scrubbers are specified.
In design of a venturi scrubber, the key parameters affect-
ing particulate collection are gas velocities and gas flow rates,
particle size distribution, pressure drop, and liquid-to-gas ratio
In addition, the following information is also required for
selection of equipment.
(a) Gas handling capacity per module.
(b) Total number of modules required.
(c) Capital investment; annual costs.
(d) Water requirement; water recirculation.
(e) Availability of the equipment or necessary downtime.
(f) Fractional collection efficiency of the device.
(g) Total power consumption as a fraction of the generated
power.
2-94
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Velocity/Gas Flow Rate—
Sizing of a venturi scrubber is often based on the inlet gas
velocity and flow rate. Usually, the inlet gas velocity is about
60 ft/sec while the inlet gas rate is dependent on the designed
scrubber diameter. Typical scrubber diameters are under 10 ft.
If the gas rate is too high to be accommodated by one scrubber,
several scrubbers should be designed for the system.
The gas velocity through the venturi will decrease more at
higher temperatures than at lower temperatures. Also, at higher
temperatures higher liquid-to-gas ratios are required for heat
transfer.
L/G Ratio—
The L/G ratio typically ranges from 1 to 15 gpm/1000 acf for
all of the scrubbers discussed and is basically a function of
inlet gas temperature, inlet solids content, and method of water
introduction. At higher inlet gas temperatures, evaporation of
the scrubbing liquor may occur at the point of gas/liquid con-
tact. Where inlet dust loading is heavy, the L/G ratio should be
increased to minimize solids buildup and plugging of drains.
Although pressure drop across the venturi is essentially inde-
pendent of specific design, the less efficient methods of water
introduction will require additional scrubbing liquor to meet
efficiency requirements.
Pressure Drop—
Design for a given application requires consideration of
throat velocity and L/G ratio to achieve the maximum collection
2-95
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efficiency for the energy spent. The energy spent is often
indicated by the gas pressure drop across the scrubber, which
ranges typically from 10 to 20 in. H_0 for venturi scrubbers, 2
to 6 in. for impingement scrubbers, 1 to 2 in. for packed towers,
and 0.05 to 1 in. across a 4-in. bed in a mesh mist eliminator.'
Achieving a given pressure drop depends on the relationships
between throat velocity and L/G ratio. Actually, only one set of
conditions will yield the maximum efficiency for the energy
spent. That one set of conditions is the only one that will
create maximum droplet surface with a minimum L/G ratio during
atomization.
Particle Size Distribution--
The particle size distribution in the inlet gas stream, a
key factor in control equipment design, often varies with process
operating conditions. Data on fractional collection efficiency
for submicron particles are particularly difficult to obtain.
When one is speaking of greater collection efficiencies, it.
should be clear that this means increased fractional collection
efficiencies in the submicron particle size range.
Materials Selection--
Because scrubber slurry and the gas streams often carry
abrasive solids that can erode, selection of construction
materials must be considered. Where abrasion-resistance require-
ments exceed the limits of stainless steel, the designer may
select fiberglass reinforced plastic (polyester). Abrasion-
resistent liners are also needed to withstand high temperatures.
2-96
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Mist Eliminator--
Mist eliminators are necessary to control undesirable emis-
sions of liquid droplets from the scrubber, caused by atomization,
and carryover of some liquid during scrubbing.
Because of the solids in the scrubber liquor, entrainment of
water droplets can cause system operating problems as well as
liquid losses. Suspended or dissolved particles can cause solids
buildup, and suspended solids can cause erosion. Among the many
problems caused by buildup of solids is the resultant increase in
pressure drop.
2.5.3 Correlations
Another difficulty in designing venturi scrubbers, impinge-
ment scrubbers, packed towers, and shower mesh demisters is the
unavailability of reliable design models. Nearly all of the
published scrubber models show either by formulas or by design
curves that the empirical correlations are based on inertial im-
paction or overall power input. Usefulness of empirical models
is often limited because both control and process variables vary
within certain ranges. Therefore, extrapolating design data from
empirical models involves some risk. Moreover, particulate col-
lection in all four types of scrubbers depends not only on
particle size distribution but also on particulate properties,
such as specific gravity, wetability, agglomeration, and solu-
bility in the scrubber liquid.
2-97
-------
32
The K-factor in Brink's model, f-factor in Calvert's
33 ~;4
model, and the 8 and y constants in the Power-Law model" are
all empirical constants encompassing many of the parameters upon
which particulate collection depends. Which model is suitable is
a matter of judgment. In addition, other mechanisms such as
diffusion may control collection of submicron particles.
The model used in this report was developed by Research-
Cottrell and is presented in detail in Section 4.2.2. It has
been used for the general class of gas atomized spray scrubbers.
It is specifically intended to model flooded-disc scrubbers. Al-
though the model was developed specifically for fly ash applica-
tions, we extrapolate for use here in the absence of fractional
efficiency data on kraft pulp mill and stone crushing applica-
tions. The similarity of some of the dust properties and required
pressure drops indicates that extrapolation appears reasonable.
The L/G ratio, pressure drop, and efficiencies calculated
from this model are used as a basis for capital and annual cost
evaluation, presented in Section 2.5.4. An optimum L/G ratio
was chosen, and the pressure drop increased to achieve the
desired collection efficiency.
2.5.4 Cajgital and Annual Costs of Venturi Scrubbers on Kraft
Pulp Mill _and_5_tone _Crushing Operations
Capital and annual costs are presented in Appendix A-5.
These are based on application of a flooded-disc scrubber to
collect particulate emissions. The computer model is based on
the following assumptions.
2-98
-------
The major equipment included in the venturi scrubber
system consists of a flooded disc scrubber and a mist
eliminator with a sump tank, one forced-draft fan with
driver, and two slurry pumps with drivers. The aux-
iliaries included are ductwork, expansion joints,
piping, and instrumentation.
The material of construction for both major equipment
and auxiliaries is carbon steel without linings.
The temperature of the flue gas is 400° to 500°F in lime
kilns and 200°F in smelt dissolving tanks. Control
systems applied to stone crushing operations operate at
ambient temperature.
The total capital investment for the system consists of
the major and auxiliary equipment costs, tax and freight
(average), installation costs, engineering, and con-
tingency.
The annual costs consist of:
(1) Fixed charges (at 15 percent of the total
capital investment) including depreciation, interest,
insurance, and taxes.
(2) Maintenance (at 5 percent of capital invest-
ment) including materials and labor.
(3) Labor cost (at $9 per man-hour assuming 3500
man-hours per year required).
(4) Administration (at 10 percent of labor cost).
(5) Water usage (at $0.30 per 1000 gal).
(6) Electricity usage (at $0.03/kWh)
2-99
-------
(7) Overhead cost (at 10 percent of the total cost
of water, electricity, labor, and maintenance).
6. Cost estimates are based on costs of materials and
labor in the 4th quarter of 1977.
7. Dust specific gravity is taken to be 2.4 g/cm for
kraft pulp mills and 2.7 g/cm for stone crushing
operations.
Figure 2-17 shows the system components and boundaries.
Figures A.3-1 and A.3-2 indicate the capital and annual
costs, respectively, for venturi scrubbers on sludge lime kilns.
For a system with high gas temperature and high required effi-
ciency of particulate removal, a gas precooler may be required to
reduce the effects of high temperatures on scrubber performance
as well as on cost of fans. The power consumption of the fan has
a considerable effect on the total operating cost in high-pressure-
drop operations.
Figures A.3-3 through A.3-6 present capital and annual costs
of venturi scrubbers applied to non-salt and salt laden bark/
fossil fuel fired boilers respectively. The presence of salt
in the fuel can severely limit the efficiency of the venturi
scrubber at the maximum estimated pressure drop of 20 inches of
water. This reduced efficiency has been confirmed by field tests
of a venturi scrubber at similar pressure drops and L/G ratio
as the data presented here.
The result of having salt in the fuel is that capital and
annual cost for the salt laden venturi scrubbers are higher
2-100
-------
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O
CO
E
CD
4-J
to
>^
w
14
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U
tfi
U
U)
• •-I
T3
T3
CJ
13
O
O
C
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I
CM
2-101
-------
than the non-salt scrubbers, but far less removal of particulate,
is provided.
Since the particles emitted from crushers are coarser than
those from conveyors, a lower pressure drop will effect a higher
collection efficiency (compare Figure A.3-7 with Figure A.3-9).
Figures A.3-8 and A.3-10 show clearly the strong effect of elec-
tric power consumption on total operating costs of both crushers
and conveyors. In high-efficiency operations, nearly all of the
power consumption is attributed to the fan. The fixed charges,
such as equipment depreciation, interest, taxes, and insurance,
are considerably smaller than the cost of fan operation. Labor,
maintenance, water usage, and overhead are grouped into the
"others" category.
2.6 FABRIC FILTERS
2.6.1 System Characteristics
Within the kraft pulping operation, the application of
fabric filtration for control of particulate emissions is limited
to bark or bark/fossil fuel-fired power boilers. The design
parameters discussed in this section are derived from three known
baghouse installations at two mills, Simpson Timber Company in
Shelton, Washington, and Long Lake Lumber Company in Spokane,
Washington. A fourth baghouse system has recently been purchased
by a Georgia-Pacific mill in Bellingham, Washington, but is not
yet installed. Table 2-20 lists the pertinent design criteria
for power boiler applications based on two of the currently
operational systems and the proposed new unit.
2-102
-------
Table 2-20. DESIGN PARAMETERS FOR KRAFT PULP
POWER BOILER BAGHOUSES
Volume flow rate, acfm
Inlet gas temoerature,
Op
A/C ratio, ftVacfm
Bag cleaning method
Pressure drop, in. H_0
Bag fabric
Precollector
Material handling
system
Simpson
Timber
230, 000
500
A . 5
Pulse jet
9-9. 5
Teflon
coated
fiberglass
Mechanical
cinder
collector
Screw con-
veyor
Long Lake
Lumber
25,000
400
4.0
Pulse jet
5.8-6.8
Nomex
None
Screw con-
veyor
Georgia-Pacific
180, 000
440
4.0
Pulse jet
a
Teflon coated
fiberglass
None
Screw conveyor
Collector not yet installed.
2-103
-------
Fabric filtration is the preferred method of particulate
control in the crushed stone industry because of the dry, inert
mature of the emitted dust collected at ambient conditions.
Also, since the captured dust is often used as a stone product or
is recycled within the plant, baghouses are ideally suited to
this industry. The collectors used to control the various unit.
operations employ the same basic design parameters and usually
differ only in size depending on the number of ventilation points
in the system. Typical design parameters for baghouses serving
drilling, crushing, screening transfer operations are shown in
Table 2-21.s
Data gathered by the EPA during emission tests on the 12
baghouse units (listed in Table 2-21) used to control a variety
of rock types, including limestone, traprock and cement rock, in-
dicated that the size distribution of particulates collected, the
rock type processed, and the facility controlled do not sub-
stantially affect baghouse performance.
Parameters important to fabric filter system design include
air-to-cloth ratio, pressure drop, cleaning mode and frequency of
cleaning, composition and weave of fabric, degree of sectionali-
zation, type of housing, and gas cooling. Baghouses are rela-
tively insensitive to process variables such as chemical compo-
sition (providing that the correct bag fabric is chosen) , part-
icle size, electrical resistivity, etc.; thus, there tends to be
very little substantial design difference from one application,
or indeed from one manufacturer, to the other, when comparing
2-104
-------
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2-105
-------
baghouses with the same cleaning mechanism. Differences that do
exist are generally related to maintenance (e.g., number of bag
rows accessible from a given interior walkway; method of bag cuff
attachment to cell plate; etc.). Pertinent baghouse design
criteria are briefly discussed below.
Air-to-Cloth Ratio—
A major factor in the design and operation of a fabric
filter, the air-to-cloth (A/C) ratio, is the ratio of the quanti-
ty of gas entering the filter (ft /min) to the surface area of
2 3
the fabric (ft ). The ratio is therefore expressed as ft"/min
per ft , or sometimes also as filtering velocity (ft/min). Most
often only the first member of the ratio term is given, e.g., an
3 2
A/C ratio of 1.5 implies 1.5 ft /min per 1.0 ft . In general,
lower ratios are used for filtering of gases containing small
particles or particles that may otherwise be difficult to cap-
ture. Selection of the ratio is based on industry practice or
the recommendation of the filter manufacturer.
Design A/C ratios for power boiler fabric filters are about
4/ or 5/1. The three units in operation have A/C ratios of 4/1,
4.8/1, and 4.5/1. The new Georgia-Pacific baghouse will have a
4/1 net A/C ratio. Most of the particulate emitted from bark
boilers tends to be in the submicron to 10 urn size range. Thus,
enough of the larger particles are present to allow use of a
fairly high A/C ratio, but the amount of fines in the mix pre-
vents the A/C ratio from being raised much further. Higher
ratios may result in excessive pressure drops. An example of
2-106
-------
this problem is seen at one of the installations at Simpson
Timber. The pressure drop is 3 in. above design and the par-
ticulate size distribution is skewed largely toward the submicron
range (much of the particulate consists of NaCl from the sea-
soaked bark).
Design A/C ratios for fabric filters on crushed stone opera-
tions range from 5.0/1 to 7.0/1 for pulse jet units and 2.0/1 to
3.1/1 for shaker units.
Pressure Drop--
Pressure drop in a fabric filter is caused by the combined
resistances of the fabric and the accumulated dust layer. The
resistance of the fabric alone is affected by the type of cloth
and the weave; it varies directly with air flow. The permeability
of various fabrics to clean air is usually specified by the manu-
3 2
facturer as the air flow rate (ft /min) through 1 ft of fabric
when the pressure differential is 0.5 in. H_0 in accordance with
the American Society of Testing and Materials (ASTM). At normal
filtering velocities, the resistance of the clean fabric is
usually less than 10 percent of the total resistance. The
spaces between the fibers are usually larger than the particles
that are collected. Thus the efficiency and low-pressure drop of
a new filter are initially low. After a coating of particles is
formed on the surface, the collection efficiency improves and the
pressure drop also increases. Even after the first cleaning and
subsequent cleaning cycles, collection efficiency remains high
because the accumulated dust is not entirely removed.
2-107
-------
The pressure drop through the accumulated dust layer has
been found to be directly proportional to the thickness of the
layer. Resistance also increases with decreasing particle size.
Even though several studies have been devoted to filtration
theory, it is difficult to relate collection efficiency and
pressure drop on an industrial scale.
The range of operating pressure drop for bark boiler bag-
houses is 6 to 12 in. H.,0, and it is preferable to operate at
the lower end of this range.
Pressure drops across fabric filters in crushed stone opera-
tions from respondents in this study, fall in the range of 2 to 8
in. HO. Pressure drops are normally higher in collectors used
on tertiary crushing and screening operations than in those used
on primary crushing because of the smaller particle sizes en-
countered.
Cleaning of Fabric Filters--
Table 2-22 shows various cleaning methods that are used to
remove collected dust from fabric filters to maintain a nominal
pressure drop of 2 to 6 in. H_0. A discussion of the operation
of various cleaning methods is presented in Section 3.3.
Each method has advantages and disadvantages. For example,
when reverse air is used with a ringless bag, it creases the bag
considerably and narrows the cross sectional area available for
reverse flow to occur. This can be alleviated by use of anti-
collapse rings. The mechanical shaker is usually applied in a
horizontal fashion to minimize flex abrasion (fiberglass bags),
2-108
-------
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2-109
-------
which shortens the life of glass fabric. The pulse jet method,
which uses jets and short bursts of compressed air, dislodges the
dust from the bag wall during filtering to eliminate off-line
cleaning.
The reverse air method is the most gentle of the three
methods discussed above, and it promotes longer bag life. How-
ever, this method alone may not provide adequate cleaning and may
be used in combination with shake or pulse jet (with less pres-
sure) methods to increase the effectiveness of cleaning and
minimize bag wear.
All three baghouse installations on bark boilers use pulse
jet as the cleaning mechanism. Simpson Timber Company initially
used reverse air and pulse jet for cleaning at its Shelton,
Washington, baghouse, but discontinued the reverse air after
finding it to be ineffective. Installations in the crushed stone
industry use pulse jet and shaker mechanisms in about equal
numbers.
Frequency of Cleaning--
The cleaning cycle should be as short as possible so that no
sizable portion of the total fabric will be out of service at any
given time. With shake cleaning equipment, for example, a common
ratio of cleaning to deposition time is 0.1 or less. With a
ratio of 0.1, 10 percent of the compartments in the baghouse are
out of service at all times during operation. Therefore, the
frequency of cleaning should be designed to minimize this ratio.
As mentioned previously, with pulsing equipment, the cleaning
2-110
-------
time is very brief, yielding a very low ratio of cleaning time to
filtering time.
Selection of Fabric--
Selection of fabric is generally based on the operating tem-
perature and on resistance of the fabric to abrasion and corro-
sion. Table 2-23 shows typical characteristics of various fabrics,
which include cotton, wool, fiberglass, and other man-made
fibers. Many fabric weaves are available; or the fabric may be
felted, a process whereby the identity of the separate yarns
tends to be replaced by a more uniform mat. Felted fabrics are
almost always cleaned by reverse jet or pulse jet methods.
Fabric characteristics may also be altered by further treatment
for specific purposes, such as to decrease adhesion or improve
wearability. Silicones are often used on fiberglass to reduce
abrasion.
Since inlet gas temperatures on kraft pulp mill power
boilers range as high as 500°F, the bag fabric must be heat-
resistant. The two Simpson Timber units contain Teflon B-coated
fiberglass bags for operation at 500°F. The Long Lake baghouse
uses Nomex bags and operates at a temperature of about 410°F.
The new unit at Georgia-Pacific will use Teflon coated fiberglass
for operation at 440°F. The preferred cleaning mechanism for the
collector is pulse jet (flex cleaning), which allows for maximum
filtering time and minimum interruption of the filtering flow.
It also allows for a slightly smaller-sized collector.
At temperatures below 275°F, as is the case in the crushed
stone industry, polyester is preferred.
2-111
-------
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2-112
-------
Degree of Sectionalization--
Design of fabric filter sectionalization (the number of
separate filter compartments) requires knowledge of the variation
in gas flow with respect to process or plant ventilation, the
sizes of commercially available units, and the expected frequency
of maintenance. Individual compartments in small collectors
2
may contain as little as 100 ft of fabric surface, although some
large units with a capacity of 50,000 ft /min may have only one
compartment. Except in reverse jet and pulse jet units, at
least one compartment in any collector is out of service during
the cleaning cycle.
Filter Housing--
Configuration of the filter housing depends on the required
fabric surface area and on the temperature, moisture content, and
corrosiveness of the gases. When the baghouse is designed so
that the dirty gas enters the inside of the bags under positive
pressure, housing may be needed only for weather protection or
for emission measurements.
The floor area required for baghouses depends on the filter-
ing surface area, size of the bags, and spacing between bags.
2
For example, 1750 ft of filtering area can be provided in about
2
80 ft of floor area by using bags 6 in. in diameter and 10 ft
long, and allowing 4 in. between bags. If 12-in.-diameter bags
are used, they must be about 14 ft long to provide the same
filtering area in the same floor space, though 12-in.-diameter
bags can easily be obtained in lengths of 20 ft or more when
2-113
-------
there is adequate head room. This configuration (12 in. x 20 ft)
2
would provide a baghouse having about 2500 ft of filtering area
in the same floor space (80 ft ). Because the length/diameter
ratio affects the stability of vertical bags, care must be taken
to ensure that bags do not rub together during operation or
cleaning. In general the length/diameter ratio ranges from 5 to
40, but more commonly is between 10 and 25. Respondents in
this study indicated a range of 17 to 31 for the length/diameter
ratio.
Design consideration must be given to adequate space for the
collecting hopper below the filter bags. Hoppers are commonly
designed with 45-degree or 60-degree sloping sides to provide
adequate sliding, and with some dusts a 70-degree slope is re-
quired. Dust collected in the hopper can be removed by screw
conveyors, rotary valves, trip gates, air slides, and other
methods.
The most common construction material for the housing is
steel; other materials, such as concrete and aluminum, are also
used. Corrugated asbestos cement paneling is often used for the
exterior roofing and siding of the housing in combination with
37
interior walls and partitions of steel.
Gas Conditioning or Cooling--
Exhaust gases are often too hot to go to the baghouse immed-
iately, so they are cooled before entering the filtration system.
Gas cooling generally is not required for kraft pulp mill power
boilers, because most are equipped with air preheaters. Likewise
crushed stone processes do not require gas cooling.
2-114
-------
2.6.2 Correlations
As stated earlier, it is difficult to establish correlations
between fabric filter design parameters and specific processes
because the fabric filters used in the various applications are
very similar. Thus only capital and annual costs are presented
here.
2.6.3 Capital and Annual Costs of Fabric Filters
Bark/Fossil Fuel-.ired Boilers--
Only minimal data are available to define capital and annual
costs for fabric filtration systems serving pulp mill power
boilers. The only reliable cost information was provided by
Simpson Timber Company. Their two fabric filters, having capa-
cities of 100,000 cfm and 130,000 acfm with a total cloth fil-
tration area of 55,315 ft , were purchased and installed at a
cost of $1.9 million in 1976. This cost is equivalent to $8.25/
acfm or $34.35/ft2 of cloth.
The annualized cost of operation of the two collectors could
not be isolated from other plant operation disbursements, but the
annual maintenance costs are estimated to be about $75,000. Most
of this expense was related to bag replacement (600 bags changed
in the past year) .
Crushed Stone Industry—
Deriving typical costs of a fabric filtration control system
in crushed stone operations is complex. Each plant represents
unique problems in terms of materials, equipment, plant layout or
size, tonnages processed, local conditions, and air pollution
2-115
-------
control regulations. In addition, there are permanent and mobile
crushing operations.
The capital and annual cost estimates for particulate con-
trol in the crushed stone industry are abstracted from a recent
3 8
PEDCo report prepared for U.S. EPA. The three control systems
considered are fabric filters, wet suppression system, and a com-
bination fabric filter/wet dust-suppression system. Cost analyses
of these control systems are applied to three sizes of model
plants: a 200-ton/h portable plant, a 300-ton/h stationary
plant, and a 600-ton/h stationary plant. Costs represent retro-
fitted systems. These model plant sizes represent typical plant
sizes, based on industry data, and corresponding control system
design and configuration.
Installed capital costs reflect December 1977 prices. These
costs were obtained by averaging price quotations from several
39
equipment manufacturers for each system design.
Total costs of equipment for fabric filter systems include
gas cleaning equipment, fan system, hopper, and a screw conveyor
with an air lock. Direct costs represent equipment and installa-
tion costs, including foundation and support, ductwork, stack,
piping, insulation, painting, and electrical work.
Costs of equipment for wet dust-suppression systems include
the dustsuppression equipment, water filter and flush, high-
pressure truck-dump station, shelter house, and equipment winter-
ization. Direct costs include equipment, foundation and supports,
4 o
piping, insulation, and electrical work.
2-116
-------
Costs of other equipment for fugitive dust control are
itemized separately. Indirect costs include engineering, con-
struction and field expenses, construction fees, startup, per-
formance test, and contingencies. Total capital cost is the sum
of direct and indirect costs. Contingencies and retrofit penal-
4 0
ties are included in the direct and indirect costs.
Production losses during startup as well as research and
development costs are difficult to estimate and are not included
in the capital costs.
Bases for Annual Cost Estimates--
Annual cost includes direct operating costs and fixed costs.
Direct operating cost components are power, water, maintenance,
operating labor, and supplies. Fixed cost components are pay-
roll, indirect costs, insurance, taxes, and capital recovery.
Return on investment and product recovery credits are in-
significant and therefore are not included in annual costs.
Several equipment manufacturers estimated the annual costs,
4 0
which reflect December 1977 prices. Depreciation and interest
on the capital investment are computed by means of a capital
recovery factor that is dependent on the operating life of the
equipment and the current interest rate. Unless otherwise
stated, an operating life of 15 years and an annual interest rate
of 10 percent are assumed to yield a capital recovery factor of
13.2 percent of the capital costs.
Table 2-24 presents the bases for computing annualized costs
for most cost components.
2-117
-------
Table 2-24. ANNUAL COST COMPONENTS FOR FABRIC FILTER
CONTROL SYSTEM38
Cost component
Basis
Direct operating costs
Utilities
Water
Electricity
Operating labor
Direct
Supervision
Maintenance and supplies
Labor and materials
Supplies
Fixed costs
Plant overhead, payroll,
taxes, insurance
Capital recovery; 15 years at
$0.25/1000 gal
$0.04/kWh
$10/man-hour
15% of direct labor
6% of capital costs
15% of labor and materials
4% of capital cost
13.2% of capital cost
Fabric Filter Control System—
Model plant parameters—Discrete emission point-sources
(sizing and transfer operations) controlled by fabric filtration
are conveying, packaging, screening, milling, and pulverizing
operations.
Figure 2-18 indicates the varieties of combined exhaust flow
rates from point sources in proportion to plant capacity. Table
2-25 presents process parameters and emission characteristics of
the three model plants.
2-118
-------
1C-
o
TO4
10?
2 3 4 56789 103
PLANT CAPACITY, tons/h
Figure 2-18. Exhaust gas volumes at various plant capacities.
39
2-119
-------
Table 2-25. CHARACTERISTICS OF EXHAUST GAS FROM MODEL SIZING
AND TRANSFER OPERATIONS39/40
Plant size, tons/h
Gas flow rate, acfm
Temperature, °F
Moisture content, %
Dust loading
Inlet, gr/scf
Outlet, gr/scf
Inlet, Ib/h
Outlet, Ib/h
Cleaning efficiency, %
200
33,000
70
2
10
0.0222
2830
62
99.78
300
48,000
70
2
10
0.0222
4110
90
99.78
600
82,000
70
2
10
0.0222
7030
155
99.78
Because of their layouts, the plants that have capacities of
200 tons/h and 300 tons/h each require two separate fabric filters,
whereas the largest plant that has a capacity of 600 tons/h re-
quires three fabric filters.
Design specifications for each of the fabric filters are as
follows:40
Type: pulsed jet, negative pressure
Filter velocity: 6.5 ft/min
Filter media: polypropylene felt bags
Construction: carbon steel housings
Collection efficiency: 99.78 percent
Pressure drop: 17 in. H_0
Control Costs—Table 2-26 presents capital and annual costs
of fabric filter systems serving the model plants. The total
2-120
-------
Table 2-26. CAPITAL AND ANNUAL COSTS OF FABRIC FILTER SYSTEMS
FOR MODEL SIZING AND TRANSFER OPERATIONS40
Exhaust flow rate, acfm
Number of units
CAPITAL COST
Equipment
Direct
Indirect
Capital
Total capital cost '
ANNUAL COST
Direct operating
Fixed
Total annual cost per
unit
Total annual cost
COST-EFFECTIVENESS
£/lb pollutant removed
Plant size, tons/h
200
16,500
2
$ 29,300
76,800
16,000
92,800
$186,000
$ 11,700
16,000
$ 27,700
$ 55,400
1.2
300
24,500
2
$ 38,700
101,300
19,200
120,500
$241,000
$ 15,500
20,700
$ 36,200
$ 72,400
1.1
600
27,300
2
$ 42,600
111, 900
21,300
133,200
$400,000
$ 17,800
22, 900
$ 40,700
$122,000
1.1
Flow rate for individual sources being controlled.
Capital cost of each unit multiplied by number of units.
All costs escalated to December, 1977, using Chemical Engineer-
ing Plant Index.
Based on 2200 hours of operation per year at 75 percent of
rated capacity.
2-121
-------
capital costs of retrofitting the fabric filter systems on the
200-tons/h, 300-tons/h, and 600-tons/h model plants are $186,000,
$241,000, and $400,000 respectively. The equipment cost repre-
sents approximately 32 percent of the total capital costs. The
capital cost, expressed as cost per unit volume, ranges from
$4.88 to 5.64 per acfm for the large to the small plant, as a
result of economy of scale.
The annual costs are estimated to be $55,400, $72,400, and
$122,000 for the small, medium, and large model plants, respec-
tively. These costs are based on 2200 hours of operation per
year at 75 percent of the rated capacity. The direct operating
costs represent about 43 percent of the total annual costs.
Cost-effectiveness--Cost-effectiveness does not differ
significantly for each plant size because annual costs and the
amount of pollutant removed are fairly proportional to exhaust
volume. The cost is computed at about l.lC/lb of pollutant re-
moved. Figure 2-19 illustrates the variation of cost-effective-
ness with plant size. This illustration indicates that cost-
effectiveness will improve at some point between a 200- to 300-
tons/h plant. These cost estimates are applied to the model
plants only, however, and cannot be used for general estimating.
For plants of less than 200 tons/h cost could exceed 1.2C/lb
of pollutant removed.
Wet Dust-Suppression System—
Model plant parameters—Dust emissions at critical dust-
producing points in the process flow are controlled by a wet
dust-suppression system.
2-122
-------
The wet dust-suppression systems applied to each model plant
40
include the following auxiliary items:
Shelter house for pump metering mechanism
Water filter and flush system
System winterization
Automatic spray at truck dump station.
Control costs—Table 2-27 presents capital and annual costs
of the wet dust-suppression system for each model. Capital costs
for the 200-tons/h, 300-tons/h, and 600-tons/h model plants are
estimated at $71,000, $73,600, and $80,400, respectively. The
equipment costs represent about 35 percent of the total capital
costs. Capital costs do not increase rapidly with size because
some of the equipment is identical for each model plant. Hence,
the cost per unit size decreases rapidly for wet dust-suppression
of a fugitive dust source.
Annual costs are estimated at $15,000, $16,000, and $20,100
for the 200-tons/h, 300-tons/h, and 600-tons/h model plants,
respectively. These estimates are based on 1650 hours of opera-
tion per year. As shown in Table 2-27, the direct operating
costs vary more in proportion to the plant size than do the fixed
costs.
The cost-effectiveness of wet dust-suppression systems in-
creases rapidly with plant size. The actual cost-effectiveness
estimate cannot be determined because no estimate of emissions is
available.
2-123
-------
Table 2-27. CAPITAL AND ANNUAL COSTS OF WET DUST-SUPPRESSION
SYSTEMS FOR CRUSHERS, SCREENS, TRANSFER POINTS, AND
CRUSHER FEEDS40
CAPITAL COST
Equipment
Total direct cost
Total indirect cost
Total capital cost
ANN'JAL COSTb
Direct operating
Fixed
Total annual cost
Plant size, tons/h
200
$24,200
62,900
8,100
$71,000
$ 2,900
12,100
$15,000
300
$26,300
65,200
8,400
$73,600
$ 4,100
12,700
$16,800
600
$28,600
71,600
8,800
$80,400
$ 6,400
13,700
$20,100
Includes dust-suppression equipment, water filter and flush,
high-pressure truck-dump station, shelter house, and equipment
winterization.
Based on 1650 hours of operation per year.
Combined Fabric Filtration and Wet Dust-Suppression--
Effective dust control can be achieved at some plants by
applying a combination of fabric filtration and wet dust-suppres-
sion systems. This strategy probably is more economical than
complete dry collection because fabric filter systems are much
more expensive. A combined system would provide greater emission
reduction than complete wet dust-suppression. The cost-effective-
ness would be much greater than for fabric filter systems alone.
Model plant parameters—Fabric filters of the type previously
discussed are applied at points where fine particle size emissions
occur in the model plants, primarily at secondary and tertiary
crushers and screens. Wet dust-suppression techniques are used
2-124
-------
at primary crushers, screens, transfer points, and crusher feeds,
where particle sizes are much larger and moisture content is
higher.
Fabric filter costs are based on estimated exhaust gas flow
rates obtained from Reference 41 for each model plant.
Control costs—Table 2-28 presents capital and annual costs
for control by the separate and combined systems. The total
capital costs of the combined system for the 200-tons/h, 300-ton/h,
and 600-tons/h model plants are estimated to be $135,000, $159,000,
and $195,000, respectively. The corresponding annual costs are
estimated to be $35,500, $43,400, and $55,500. The capital and
annual costs are considerably lower for the combined system than
for a complete fabric filter system. (See Table 2-26.) The
costs increase less rapidly with plant size than with the com-
plete fabric filter control.
Cost-effeetiveness—Because no estimate can be made of the
emission reduction achieved by the wet dust-suppression system,
its cost-effectiveness cannot be computed. Assuming a propor-
tional increase of emissions with plant size, the cost-effective-
ness would be in ratios of about 17:14:9 for the small, medium,
and large plants, respectively.
2-125
-------
Table 2-28. CAPITAL AND ANNUAL COSTS OF COMBINATION
FABRIC FILTERS AND WET DUST-SUPPRESSION SYSTEMS FOR CRUSHERS,
SCREENS, TRANSFER POINTS, AND CRUSHER FEEDS39
A.
CAPI
ANN!'
B.
Fabric filter
Exhaust flow rate, acfm
Number of units
TAL COST
Equipment
Direct
Indirect
Total capital cost
AL COST
a
Direct operating
Fixed
Total annual costs
Wet dust suppression
CAPITAL COST
AN'N'L
Equipment
Direct
Indirect
Total capital costs
AL COST
b
Direct operating
Fixed
Total annual costs
Total combined
capital costs
Plant size, tons/h
200
11,000
1
$ 21,300
56,500
13,900
$ 70,400
$ 8,500
12,200
$ 20,700
$ 21,800
56,600
7,300
$ 63,900
S 2,900
10, 900
13,800
$134,000
300
16,500
1
S 28,300
76,800
15,000
$ 92,300
$ 11,900
15,000
$ 26,900
$ 23,700
58,700
7,600
$ 66,300
5 4,100
11,400
15,500
$159,000
600
25,000
1
$ 40,000
104,500
19,200
5124,000
$ 15,500
21,300
$ 36,800
$ 25,700
64 ,400
7,900
$ 71,300
$ 6,400
12,300
18,700
$195,000
3 Based on 2200 hours of operation per year at 75 percent of
rated capacity.
Based on 1650 hours of operation per year.
2-126
-------
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REFERENCES - SECTION 2
1. Standards Support and Environmental Impact Statement -
Volume 1: Proposed Standards of Performance for Kraft Pulp
Mills. EPA-450/2-76-014-a. September. 1976.
2. Atmospheric Emissions from the Pulp and Paper Manufacturing
Industry. EPA-450/1-73-002. September 1973.
3. Environmental Pollution Control: Pulp and Paper Industry,
Part I - Air. EPA-625/7-76-001. October 1976.
4. Weant, G.E. Characterization of Particulate Emissions from
the Stone-Processing Industry. Research Triangle Institute,
EPA Contract No. 68-02-0607-10. May 1975.
5. Standards Support and Environmental Impact Statement - An
Investigation of the Best Systems of Emission Reduction for
Quarrying and Plant Process Facilities in the Crushed and
Broken Stone Industry. EPA/OAQPS/RTP. August 1975.
6. Particulate Pollutant Systems Study: Volume III - Handbook
of Emission Properties. Midwest Research Institute. EPA
Contract No. 22-69-104. May 1971.
7. Electrostatic Precipitator Newsletter (September 20, 1977).
8. Federal Register; Standards of Performance for New Sta-
tionary Sources, Kraft Pulp Mills. Part II. September 24,
1976.
9. Oglesby, S. and G.B. Nichols. A Manual of Electrostatic
Precipitator Technology, Part I: Fundamentals. p. 204. PB
196 380. 1970.
10. Matts S. and P.O. Ohnfeldt. Efficient Gas Cleaning with SF
Electrostatic Precipitator. Fla'kt, AB Svenska Flakt Fabricken,
June 1973.
11. Vandegrift, A.E., et al. Particulate Pollutant System
Study, Volume III, Handbook of Emission Properties. Midwest
Research-Institute. Kansas City, Missouri. 1971.
12. Marchello, J.M. and j.j. Kelly (eds.) Gas Cleaning for Air
Quality Control, p. 219. Marcel Dekker. New York. 1975.
2-128
-------
13. Deutsch, W. Ann. Physik 68. 1972.
14. Feldman, P.L. The Effect of Particle Size Distribution on
the Performance of Electrostatic Precipitators. Presented
at the 68th Annual Meeting of APCA, No. 74-02.3, (June
15-20, 1975).
15. Myron Robinson, W. Strauss (Ed.), Electrostatic Precipita-
tion, in Air Pollution Control. Volume II. Wiley-Inter-
science, New York, 1972.
16. Cooperman, P. Trans. Am. Inst. of Elec. Engrs. Vol. (79)1.
(1960).
17. Cooperman, P., Research-Cottrell, Inc., In-house Report.
18. Bump, R.L. Precipitator Design for Low-Odor Boilers Offer
Special Problems. Pulp and Paper. 1976.
19. Oglesby, S. and G.B. Nichols, A Manual of Electrostatic
Precipitator Technology, Part II: Application Area. p. 345.
PB 196 381. 1970.
20. Paul, John E. Application of ESP for Control of Fumes from
Low-Odor Pulp Mill Recovery Boilers. JAPCA Vol. 25, No. 2.
1975.
21. Cheremisinoff, P.N. and R.A. Young, Air Pollution Control
and Design Handbook. Part I, Chapter 10. 1977.
22. Stern, A.C. Air Pollution. Third Edition, Volume IV,
Chapter 3, p. 106. 1972.
23. Reference 22, p. 120.
24. Sittig, Marshall. Pulp and Paper Manufacturing, Energy
Conservation and Pollution Prevention. p. 370. Noyes Data
Corp., Park Ridge, N.J. 1977.
25. Reference 24, p. 373.
26. Steenberg, L.R. Air Pollution Control Technology and Cost in
Seven Selected Emission Sources. IGCI. EPA Contract No.
450/3-74-060. December 1974.
27. Mcllvaine Wet Scrubber Manual, Volume 1, Chapter III, p. 58.
28. Reference 27, p. 8.
29. Reference 24, p. 373.
30. Reference 27, p. 47.
2-129
-------
31. Mcllvaine Wet Scrubber Manual, Volume I, Chapter III.
32. Brink, J.A. and C.E. Constant, 2nd, Eng. Chem. 50 (8).
33. Calvert, S.D., Lundgren, and D.S. Mehta, JAPCA 22 (7), 1972,
p. 529.
34. Semrau, K.T. JAPCA, IQ_ (3) 1960, p. 200.
35. Gorman, P.E., A.E. Vandegift, and L.J. Shannon. Fabric
Filters in Gas Cleaning for Air Quality Control. Marchello,
J.M. and J.J. Kelly (eds.). Marcel Dekker, Inc., New York.
1975.
36. McKenna, J.D., J.C. Mycock, and W.O. Lipscomb. Applying
Fabric Filtration to Coal-Firing Industrial Boilers - A
Pilot Scale Investigation, EPA-650/2-74-048-a, August 1975.
37. Billings, C.E., and J. Wilder. Handbook of Fabric Filtra-
tion Technology, Volume 1. GCA Corporation. Contract No.
CPA-22-69-38, December 1970.
38. Crushed and Broken Stone Industry Cost Technology Guidelines
Document (CTGD), draft report, prepared by PEDCo Environ-
mental, Inc. for U.S. EPA Economic Analysis Branch, Research
Triangle Park, North Carolina. April 1978.
39. The Crushed Stone Industry: Economic Impact Analysis of
Alternative Air Emission Control Systems. Arthur D. Little,
Inc. EPA Contract No. DU-AQ-76-1349. Final Draft. Septem-
ber 1975.
40. Nonmetallic Minerals Industries Control Equipment Costs.
Industrial Gas Cleaning Institute, Stamford, Connecticut,
EPA Contract No. 68-02-1473. February 1977.
41. Evans, R.J. Methods and Costs of Dust Control in Stone
Crushing Operations. Bureau of Mines Information Circular
No. 8669. U.S. Department of the Interior. 1975.
2-130
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SECTION 3
OPERATION AND MAINTENANCE OF PARTICULATE CONTROL DEVICES
As with other complex equipment, the successful functioning
of pollution control systems depends not only on sound design
and proper installation, but also on proper operation. Ideally,
the plant personnel who use and maintain the equipment will
understand the engineering principles on which the control
system is based and will apply this knowledge both in routine
operation/maintenance and in emergency situations.
3.1 OPERATION AND MAINTENANCE OF ELECTROSTATIC PRECIPITATORS
This section deals specifically with electrostatic precip-
itator applications on recovery furnaces and bark/fossil fuel-
fired boilers used at kraft pulp mills.
Problems with electrostatic precipitators can arise when the
precipitator is brought on line and also after extended operation.
Since the possible causes of poor precipitator performance are
diverse, it is impractical to outline a single procedure for
determining the nature of a specific problem. When a malfunction
occurs, the operator must depend on his theoretical understanding
of the equipment, backed by his practical experience. This sec-
tion of the report, therefore, provides background information on
precipitator operation, together with detailed maintenance and
trouble-shooting procedures for the major component categories.
3-1
-------
Presently, two methods are used in recovery boiler operations
to concentrate the black liquor: 1} the more prevalent conven-
tional method (direct-contact evaporation) and 2) the low-odor
method (indirect contact evaporation). The nature of the parti-
culate was shown previously in Section 2.3.3, Table 2-12.
Since the basic precipitator functions are those of charging and
collection of particles, the components and controls associated
with the transformer-rectifier (T-R) sets, vibrators, and rappers
constitute the heart of the electrostatic precipitator system.
The more tenacious dust from the low-odor operation can be re-
moved from the precipitator plate by vibrating with air vibrators.
The collected dust from the conventional process is removed by
rapping; the trend, however, is to also use air vibrators on such
applications.
The procedures presented here are those suggested by Research-
Cottrell, Inc. Although other manufacturers might recommend
different procedures as dictated by details of system design,
most of the major components, and therefore the operating pro-
cedures, are similar. Where it is possible, the recommended
practices are interpreted in terms of their effects on equipment
performance.
3.1.1 Background on Precipitator Operation
Electrostatic precipitation requires two groups of equip-
ment: 1) the precipitation chajnber, in which the suspended
particles are electrified and removed from the gas, and 2) the
high-voltage transformer and rectifier, which function to create
the strong electrical field in the chamber.
3-2
-------
The chamber consists of an outside shell (precipitator shell)
made of metal or tile. Suspended within the shell are grounded
steel plates (collecting electrodes) connected to the grounded
steel framework of the supporting structure and to an earth-
driven ground. Suspended between the plates are wires (discharge
electrodes) insulated from ground, which are negatively charged
at voltages ranging from 20,000 to 105,000 volts. The great
difference in voltage of the wires and the collecting plates sets
up a powerful electrical field between them, which imparts a
negative charge to the solid particles suspended in the gas stream.
Understanding of this phenomenon requires some knowledge of
electricity and chemistry; for practical purposes it is enough
to know that the particles become electrically charged. The
negatively charged particles are attracted to the collecting plates,
which are at ground potential. The particles cling to the collect-
ing plate and become electrically inert.
The gas that enters the precipitator laden with particles
is channeled through the precipitator outlet, while the dust
collected in the hopper or drag bottom is removed via an ash
handling system.
It should be noted that reentrainment can be a problem in
the conventional process. The collected dust from a conventional
process falls into a wet bottom. The dust from the low-odor pro-
cess falls into a dry drag bottom. The cleaning mechanism can
travel in a transverse or longitudinal direction. Removal of the
collected dust (principally carbon, sand, and fly-ash) from bark-
3-3
-------
fired and combination-fired boilers is best achieved by rapping
the plates and collecting the ash in trough or pyramidal hoppers.
Figure 3-1 illustrates the major components of a kraft pulp
mill precipitator with tophousing (as opposed to insulator com-
partments) . The dust collecting and removal system will vary
depending on the application. Appendix B-l presents a more
detailed explanation of precipitator operations including subsys-
tems and components such as transformer-rectifiers, rappers,
vibrators, the upper precipitator, discharge wires, collecting
plates, and the lower precipitator. The following section
describes the fundamental operational procedures necessary for
routine operation.
3.1.2 Precipitator Startup and Shutdown Procedures
Operation of an electrostatic precipitator involves high
voltage, which is dangerous to life. Although all practical
safety measures are incorporated into the unit, extreme caution
should be exercised at all times. An electrostatic precipitator
is, in effect, a large capacitor which, when de-energized, can
retain dangerous electric charges. Therefore, grounding mech-
anisms provided at each access point should be used before
entering the precipitator.
Preoperation Checklist--
Before placing the equipment in operation, plant personnel
should perform a thorough check and visually inspect the system
components in accordance with recommendations of the manufacturer.
A complete checklist of items is presented in Appendix B-2. Some
of the major items that should be checked are summarized below:;
3-4
-------
GROUND SWITCH BOX-
ON TRANSFORMER
TRANSFORMER-
RECTIFIER
HEAT JACKET
PERFORATED-
DISTRIBUTION
PLATES
DISCHARGE
ELECTRODE
DISCHARGE
ELECTRODE
VIBRATOR
COLLECTING
ELECTRODE
RAPPERS
TOP HOUSING
ACCESS DOOR
TOP HOUSING
HOT ROOF
ACCESS DOOR
BETWEEN
COLLECTING
PLATE SECTIONS
COLLECTING ELECTRODES
WET BOTTOM
Figure 3-1.
Uet bottom electrostatic precipitator
with heat jacket.
3-5
-------
Control unit
Proper connections to controls
Silicon rectifier unit
Rectifier-transformer insulating liquid level
Rectifier ground switch operation
Rectifier high-voltage connections
High-voltage bus transfer switch operation
High-tension connection
High-tension bus duct
Proper installation
Vent ports properly installed
Equipment grounding
Frecipitator grounded
Transformer grounded
Rectifier controls grounded
High-tension guard grounded
Conduits grounded
Rapper and vibrator ground jumpers in place
Air Load Test--
After the precipitator is inspected (i.e., preoperational
check adjustment of the rectifier control and check of safety
features), the air load test is performed. Air load is defined
as energization of the precipitator with minimum flow of air (stack
draft) through the precipitator. Before introduction of an air
load or gas load (i.e., entrance of dust-laden gas into the pre-
cipitator), the following components should be energized:
Collecting plate vibrator-rappers
Perforated distribution plate rappers {if present)
High-tension discharge electrode vibrators
Bushing heaters; housing/compartments
Hopper heaters; vibrators; level indicators
Transformer rectifier (T-R)
Rectifier control units
Ventilation ar.d forced-draft fans
Dust-conveyir.g system
3-6
-------
The purpose of the air load test is to establish reference
readings for future operations, to check operation of electrical
equipment, and to detect any improper wire clearances or grounds
not detected during preoperation inspection. Air load data are
taken with the internal metal surfaces clean. The data consist
of current-voltage characteristics at intervals of roughly 10
percent of the T-R milliamp rating, gas flow rate, gas tempera-
ture, and relative humidity.
For an air load test the precipitator is energized on manual
control. The electrical characteristics of a precipitator are
such that no sparking should occur. If sparking does occur, an
internal inspection must be made to determine the cause. Usually,
the causes are close electrical clearances and foreign matter,
such as baling wire, that has been left inside the precipitator.
After the precipitator has been in operation for some time,
it may be necessary to shut it down to perform an internal inspec-
tion. At such times it is of interest to take air load data for
comparison with the original readings.
Gas Load Test—
The operation of a precipitator on gas load differs con-
siderably from operation on air load with respect to voltage and
current relationships. High current and low voltage charac-
terize the air load test, whereas low current and high voltage
characterize the gas load test. These relationships govern
operation of the precipitator and the final setting of the
electrical equipment.
3-7
-------
In general, optimum precipitator efficiencies are obtained
when the dc voltage applied to the precipitator is at the thresh-
old of sparking. The spark rate at this point will be on the
order of 50 to 150 sparks per minute and may be controlled at
this level with automatic control.
Shutdown Procedure--
To shut down the precipitator, the operator opens the con-
trol circuit start/stop switch and then opens the main circuit
breaker. Before entering the system, the operator should follow
all safety procedures. Proper grounding of all precipitator
parts is important. The key interlock system prevents access to
the interior of each T-R ground switch enclosure until the in-
dividual set is de-energized and the ground connections are made.
This system prevents access to the interior of the precipitator,
including top housing or insulator compartments, precipitator
roof doors, side doors, and hopper doors, until the T-R's of each
precipitator are de-energized and ground connections are made.
Purging the system with ambient air may also be desirable from
the standpoint of plant personnel who must inspect the internal
parts of the precipitator.
3.1.3 Inspection and Maintenance During Normal^ Operation
Appendix B-3 consists of detailed directions for plant per-
sonnel who are assigned responsibility for inspection and main-
tenance of precipitator systems. These instructions, abstracted
from Research-Cottrel1's suggested operation and maintenance
procedures, are representative of the level of inspection and
3-8
-------
maintenance required for successful precipitator operation with
minimum downtime. Although electrical portions of a precipitator
require very little maintenance, the items enumerated in Appendix
B-3 should be attended regularly if the equipment is to give
optimum service. It is considered good practice to assign one
plant operator on each shift the task of checking and recording
data on electrical equipment at the start'of the shift.
The cycle or inspection and maintenance during normal
operation includes the following components:
T-R sets and associated equipment and controls
Transformer enclosure
Pipe and guard
Vibrators
Plate rappers
Top housing
Insulator compartments
Upper high tension frame
Discharge wires
Collecting plates
Lower precipitator steadying frame
Dust collection point (dry or wet bottom)
Hoppers and screw conveyors
Precipitator shell.
Maintenance Schedule and Troubleshooting—
Appendix B-4 also presents a detailed list of maintenance
procedures that should be performed on a daily, weekly, monthly,
quarterly, semi-annual, and annual basis. These procedures
typify the level of effort required to maintain optimum operation.
For example, the annual inspection covers the following conditions
and subsystems:
3-9
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3-11
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Dust accumulation
Discharge wires
Alignment of plates and wires
High-tension and plate vibrator-rappers
High-tension frame support bushing
High-voltage electrical control cabinet
T-R sets
Dry/wet bottom
Hoppers
Screw conveyor
Table B-3-1, Appendix B-4, lists troubleshooting measures recom-
mended by precipitator manufacturers for determining the most
probable cause of precipitator problems; Table B-3-2 gives ar.
example of the frequency of failure and time to repair various
components of a typical industrial precipitator.
3.1.4 Op e r a 1 1 on a .-. d M a i n t_e_n a n c e Problems Specific to Kraft
P^lp" "Mill's" "
The information presented in the preceding sections pertains
to most precipitator applications. Table 3-1 illustrates the
important differences between utility and kraft pulp mill precip-
itators, i.e., those controlling recovery furnaces and bark-fired
and combination-fired boilers. The most common malfunctions
associated with electrostatic precipi ta tors applied to recovery
furnaces stem from corrosion and failure of rappers. Other prob-
lems result from drag bottom conveyors, plugging of the inlet
distribution plate, buildup on ladder vanes, and "snowing"
(intermittent puffing of recovery furnace stacks).
Corrosion--
Corrosion is more severe with conventional than with low-odor
recovery furnace operations, mainly because of the lower operating
gas temperatures in conventional recovery furnaces. It is
3-12
-------
essential that both the precipitator inlet and outlet flues be
insulated, as well as the shell. At flue gas temperatures below
300°F, local cold spots may cause condensation and create severe
corrosion. A heated steam or air coil system provides a means of
keeping all internal shell wall surfaces above the dew point of
vapors in the gas being cleaned and thus preventing condensation
of corrosive chemicals anywhere on the walls. Operating the wet
bottom at extremely low liquor levels leads to inleakage of cold
outside air at the agitator hubs, which causes corrosion of the
wet pan. Maintaining the proper liquor level can eliminate this
problem.
Rapper Failure--
Dust is removed from the collecting and discharge electrodes
by means of air or electrically operated vibrators/rappers. Al-
though intensities and frequencies depend on the specific installa-
tion, plate and wire vibrators and rappers are usually in con-
tinuous operation. The importance of the vibrating/rapping system
cannot be overemphasized, since any failures within the system will
cause loss of power input to the precipitator and a reduction in
precipitator performance.
Drag Bottom Conveyors—
Dust is conveyed from the electrostatic precipitator dust
chamber to the salt-cake mix tank either concurrently to the gas
flow (longitudinal drag bottom) or perpendicularly to the gas
flow (transverse drag bottom). The latter method lessens the
possibility of gas sneakage, which is typically associated with
3-13
-------
the longitudinal drag bottom. Systems that convey the dust per-
pendicular to the gas flow must be broken into at least two
sections. One conveyor mechanism should serve the inlet field
separately and should be designed to accommodate the bulk of the
dust.
Plugging of Inlet Distribution Plate and Ladder Vanes--
Plugging is a function of operating temperature. Most prob-
lems are eliminated when operating temperature exceeds 3258F.
Dust accumulations on the inlet turning vanes cause some maldis-
tribution of gas flow patterns. Any change in design to eliminate
the necessity cf these structures would improve the operation,
rair.ter.ar.ee, and performance of the precipitator.
"Snowing"—
The principal cause of "snowing" is dust-laden gases bypass-
ing the treatment zones. In wet bottom units the gases pass
through gaps between the baffle plates and tile shell. Other
causes of "snowing" can be a sudden release of particles accumu-
lated in the ductwork, too heavy vibrating or rapping, too high
gas velocities, and an increase in load on the precipitator
caused by ar. increase in throughput of the recovery furnace. Use
of a low-energy scrubber following an electrostatic precipitator
helps to eliminate "snowing."
A 1974 survey by the TC-1 corrj?ittee of the Air Pollution
Control Association (APCA) details operational problems of 36
paper mills reporting on 49 precipitators. Respondents indicated
that rapper/vibrators presented the largest maintenance problem
3-14
-------
followed by discharge electrodes, collecting plates, insulators
and dust removal system. Table 3-2 compares maintenance problems
for utility, metallurgical, and paper mill applications from the
above study. Paper mill respondents reported maintenance to be
a greater problem in general, than did utilities or respondents
from metallurgical processes.
The incidence of maintenance problems as reported by paper
mills corresponds well to the design and operational information
presented in Table 3-1 on kraft recovery boilers.
Little information is available concerning malfunctions
associated with bark-fired combination-fired boilers. One pos-
sible source of problems with such an application is the potential
for fire caused by buildup of "char" on precipitator walls and
especially in the hoppers. The fire hazard can be minimized by
installing trough type hoppers for continuous removal of dust.
Also the elimination of all inleakage will decrease the avail-
ability of air for combustion.
3.2 MAINTENANCE AND OPERATION OF MECHANICAL COLLECTORS SERVICING
BARK/FOSSIL-FUEL BOILERS
Mechanical cyclone collectors are used as primary collectors
on wood-fired boilers or as first-stage collectors for coal-
4
fired units.
Mechanical collectors separate dust from a gas stream by a
combination of centrifugal, gravitational, and inertial forces.
Cyclones, the most common of the mechanical collectors, make use
of most of these mechanical forces. Rotational action creates a
centrifugal force that drives the suspended particles to the
3-15
-------
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wall of the cylinder. The gas and the dust begin a downward
spiral towards the cone and finally to the dust discharge; this
outside downward spiral is the main vortex. As the main vortex
spirals downward, a quantity of gas is drawn radially inward to
feed a smaller inner vortex spiralling upwards to the clean gas
discharge tube.
3.2.1 Startup/Shutdown ;
Check blower rotation.
Make sure that all inspection doors, connections, etc., are
closed.
Turn on blower and check current, pressure drop, and system
pressure drop.
Check inlet and outlet gas and dust flows. To shut the
system down, pass clean air through the system to permit the cy-
clone to dry and empty out, then turn off blower.
Clean out the hoppers to prevent plugging.
3.2.2 Normal Operation
At least once a shift, the pressure drop across the collec-
tor should be recorded. Normally, the pressure drop across the
cyclone is greater than that of any other component in the sys-
tem. If the pressure drop is measured across the system and if
resistance to gas flow occurs somewhere other than in the cy-
clone, the reduction of gas flow could reduce the pressure drop
across the cyclone. It is advisable to install a manometer
across the fan and record static pressure readings from the time
of startup. These are used as a reference for subsequent opera-
tions.
3-17
-------
Every collector should be equipped with a test tap at the
inlet and outlet ducts to monitor pressure drop across the unit.
The dust discharge hopper should always be equipped with a poke-
hole on the top surface. The ash removal system is operated
frequently enough to ensure that the hopper never fills enough to
reach the bottom of the collecting tube. If there is any doubt
about this, install a bin level gauge with an alarm. Leakage
will reduce efficiency.
^ • 2 • ^ .Maintenance
Leakage into the cyclone or dust discharge hopper is diffi-
cult to detect on a negative pressure system. When a rotary
valve acts as the seal, the wear plate should be self-adjusting
so that as it wears down (metal on metal) it always maintains a
seal. When a counterweighted or flap valve acts as the seal, the
maintenance crew must check for buildup in the valve seat or
flapper plate.
Additional checks should be made on fan bearings, leakage
around gaskets and valves, and general wear and tear on the
system.
3.2.4 Operation an_d_ Maintenance Problems Specific to Cyclones
Literature regarding bark boiler operations is so scarce
that most of the design considerations for cyclones are presented
in general terms. Where information specific to bark bcilers is
available, it is included together with some typical problems of
cyclone operation.
3-18
-------
The action inside a cyclone is always dynamic; the dust
particles are either gouging and channeling or sticking to a sur-
face and then to each other, all of which can cause erosion and
dust buildup.
Erosion—
Particles with high specific gravity, and in high concentra-
tion, moving at high velocities and impinging on a surface will
erode the cyclone wall. With light inlet dust loadings, erosion
is prevalent in the cone; with heavier inlet dust loadings, ero-
sion occurs in the cylinder of the cyclone. Improper cyclone
design or poor operating conditions tend to concentrate the fast-
moving dust and increase erosion. The areas most prone to
erosion damage are weld seams, mismatched flange seams, the
bottom of the cone, and the wall opposite the inlet. Erosion can
be eliminated by installing removable wear plates of abrasion-
resistant metal or rubber. Erosion can be reduced by increasing
the diameter of the cyclone body without increasing the diameter
of the gas outlet tube. This can lead to an increase in pressure
drop. The use of troweled or cast refractory linings is the best
method of combating erosion in cyclones.
Dust Buildup—
Plugging of the cyclone reduces efficiency and increases
pressure drop, and may increase erosion. Dust buildup usually
occurs at the dust outlet or at the cyclone wall. Dust outlets
3-19
-------
become plugged by foreign matter or by hoppers overfilling.
Buildup on the cyclone wall is a function of particle size and
caking characteristics. The presence of fine dust such as that
from salt emissions from bark boilers and moisture condensing on
the walls tend to increase dust buildup. Maintaining gas veloc-
ities above 50 ft/s will minimize buildup on the cyclone wall by
the scouring action of the gas.
3.3 OPERATION AND MAINTENANCE OF WET SCRUBBERS
In the kraft pulp mill industry wet scrubbers are used on
sludge lime kilns, smelt dissolving tanks, and bark/fossil-fuel
boilers.
Historically, dry mechanical collectors and low-energy
scrubbers v.ere used to control dust emissions from the crushed
stone industry. Presently, the controls used most commonly are
fabric filters, followed in order by wet suppression techniques,
a combination of wet dust suppression and fabric filters, and
g
mechanical collectors and/or wet scrubbers. In stone crushing
operations the most co.T.nonly used wet scrubber is the venturi.
For this reason, the following commentary is restricted to gas
atc-ized spray type scrubbers.
3.3.1 Description
Gas atomized spray type scrubbers utilize a moving gas
stream to atomize liquid into drops and then accelerate the
drops. Acceleration of the gas provides impaction forces as well
as intimate contact with the liquid stream. Within this category
many differences in design and operation may be noted with re-
3-20
-------
spect to the following items: method of adjusting pressure drop
(the difference being that between the true venturi and the
annular orifice); the method of moisture elimination (centrifugal
moisture elimination, as with spinning vanes or multi centri-
fugals). In any event, most gas atomized spray scrubbers incor-
porate the converging and diverging section typical of the ven-
turi throat.
The collection efficiency of a venturi scrubber is dependent
upon the pressure drop across the scrubber. Pressure drop in
turn is dependent upon operation at close to design flow condi-
tions or provision of an adjustable venturi throat. The low flue
gas temperature and high moisture content of the flue gases from
a venturi scrubber may lead to undesirable plume rise and plume
visibility. The scrubbing medium may affect the odorous emis-
sions, and fresh water rather than contaminated condensate should
be used to minimize odorous emissions. Venturi scrubbers that
incorporate variable throat designs provide a high level of
efficiency under varying inlet conditions when operated within
their design capacity.
A typical flooded-disc scrubber system for particulate col-
lection consists of a flooded-disc scrubber, a mist eliminator
with sump, two recirculation pumps, and one booster fan or ID
fan. A gas prequencher is sometimes required for treating high-
temperature gas.
The flooded disc scrubber, shown in Figure 3-2, is composed
of a disc in the tapered throat section of a vertical hollow
3-21
-------
-"*• ~v^W~ i '" T?^~ i 5-"i -
Figure 3-2. ^.esearch-Cottrell flooded di
sc scrubber.,
3-22
-------
cylinder. The disc is supported by a pipe, and an open annulus
is formed between the wall and the disc. As the gas flows
through the annulus and the scrubbing water is ejected simultane-
ously across the disc face, atomization takes place at the an-
nulus. The millions of fine water droplets that are created are
used for particle capture in the gas stream. The particulate
collection efficiency of the scrubber depends on the degree of
atomization, which is indicated by the pressure drop across the
scrubber. Pressure drop across the scrubber can be regulated by
controlling the vertical displacement of the disc in the venturi
throat section automatically or manually. To provide the same
collection efficiency, the pressure drop must be higher in col-
lecting fine particles than in collecting coarse ones. Given
particles of the same size, the pressure drop must be higher to
achieve relatively higher collection efficiency.
The purpose of the mist eliminator is to separate the dust-
laden water droplets from the gas stream by centrifugal force.
The separated dust-laden water flows by gravity to a sump for re-
circulation to the scrubber. The solids content of the slurry
that is recirculated by the recycle pump gradually increases.
For control of the solids content level in the slurry, a purge
stream from the discharge of the recycle pump is pumped out of
the system, as indicated by the slurry density control. During
system operation, some water is lost from the sump with the
purged slurry and some is vaporized as the hot gas comes in
contact with the scrubbing liquor. To compensate for the lost
3-23
-------
water, a makeup water stream is pumped to the recirculation sump,
as indicated by the slurry level control.
All of the variables are controlled within the high and low
limits. An alarm signals any operating condition beyond the con-
trol limits to warn the operator of an abnormal condition. The
m.a;cr controls and alarms in the scrubber system include pressure
drop across the scrubber, slurry density in the scrubber, slurry
level cf the recirculation sump, and slurry flow rate to the
scrubber. For the purpose of safety, the scrubber system should
incorporate interlock circuits to protect the equipment in the
event cf an emergency.
3.3.2 Operation
Freooeration—
Before the system startup, all major items of equipment,
connecting pipes, and auxiliaries must be inspected, cleaned, and
tested. A new system should be checked for leaks and instabili-
ties by an air test for the fans and ductworks and a hydraulic
test for pipings and valves. In addition, a water test should be
cerformed to ensure that equipment, instruments, and control >'
safety systems are working properly. The items that should be
checked in preoperation tests are summarized below:
° FD/ID fan
Electrical controls, fan bearing coolant system, align-
ment, lubrication, vibration sensors, bearing tempera-
ture sensors.
3-24
-------
0 Pumps
Belt tension, pump rotation, pump alignment, lubrica-
tion, seal water, packing, pressure gauge, suction and
discharge valves, motor bearing temperature, hydraulic
system (for flooded-disc control pump).
° Control Systems
Flue gas bypass, pressure drop, makeup water rate, re-
circulation sump level, slurry density, slurry purge
rate.
° Safety Systems (interlocks and alarms)
High flue gas pressure, low level in sump, high and low
density.
° Utilities
Electric power, instrumentation air, process water,
process return water.
Startup--
To start up a system for the actual operation or for a water
test operation, one must follow the procedure described in the
system designer's operating manual. Following are several steps
in the startup of a new scrubber system:
1. Close all drain valves.
2. Turn on circuit breakers for all instruments and elec-
tric valves.
3. Set all monitoring instruments at zero reading.
4. Startup the service water system and raise the water
level in the sump to the designated level.
5. Turn on the recycle pumps circuit breakers and start up
the operating and standby pumps.
6. Turn on the circuit breaker for the disc control pump,
start up the disc control pump, and adjust the high and
low limits of the pressure drop indicator (in venturi
scrubbers) .
3-25
-------
7. Close the flue gas bypass dampers and start the fan.
8. Check the scrubber pressure controller and the system
monitoring ir.struments.
Shutdown--
Following is a general procedure for planned shutdown of a
flooded disc scrubber system:
1. Turn the flue gas damper to the bypass position and
stop the fan.
2. Close the makeup water and slurry couple valves.
3. Stop the recycle pumps (both operating and standby).
4. Open the drain valves at the slurry pumping lir.es and
flush the lir.es, gauges, and p_L.-.ps with water.
5. Stop the disc control pump and leave the disc in the
fully raised position.
6. Open the drain line on the pressure gauges to the
throat and disc and allow the line to drain.
Ncrr.al Operation-
L'nder normal operating conditions, all of the control param-
eters should be held within the defined ranges. These include
the scrubber pressure drop, recycle pur.p rate, makeup water rate,
slurry density, slurry purge rate, and recirculation sump level.
An abnormal condition is defined as a deviation of the
operating condition beyond the normal range. The abnormal con-
dition will be indicated by an alarm. If the operator cannot
correct the condition, under certain circumstances an interlock
will open the flue gas bypass damper and shut down the scrubber
system.
During normal operations the following malfunctions may
develop:
3-26
-------
1. Pump impeller wear is indicated by a reduction in
scrubber recycle flow. Valve or nozzle erosion is
indicated by an increase in scrubber recycle flow.
2. A decrease in scrubber bleed flow is associated with
line plugging. An increase in bleed flow can indicate
a worn valve.
3. An increase in pressure drop can be caused by plugging
of packing or by an increase in flow of gas or liquor.
4. An increase in pump discharge pressure at proper flow
rate usually indicates line plugging.
5. Fan inlet and outlet pressure readings can be used to
check flow as well as damper setting.
6. Dust buildup on fan blades is indicated by an increase
in fan vibration.
7. Motor current indicates whether a flow decrease is
caused by impeller wear, plugging, or an incorrect flow
meter setting. Fan current is synonymous with gas
throughput.
The following alarm conditions are associated with the
system:
1. Scrubber pressure drop
An alarm condition may occur because of a malfunction-
ing pressure drop controller, failure of the disc
control pump, jammed disc, or a rapid change of boiler
load.
2. Slurry density
An alarm condition may occur because of a malfunction-
ing control, a defect in the density control valve, a
malfunction in the sump level control, or a change in
the makeup water rate.
3. Recirculation sump level
An alarm condition may occur because of a malfunction-
ing control or because of high or low level of slurry
in the sump.
3-27
-------
4. Others
An alarm condition may occur because of plugged lines,
closed valves, pump trouble, or fan trouble.
3.3.3 Inspection and ^jJ\t_e_n_ajT_ce__Du_ring Normal Operation
Many of the items on the preoperation checklist should be
checked in routine maintenance. This maintenance generally in-
cludes unplugging lines, nozzles, pumps, etc.; replacing worn
equipment parts, erosion/corrosion prevention liners, and instru-
ments (level indicators, density indicators, etc.); and repairing
damaged components when this is practical from the standpoint of
labor and materials.
The wet-dry line must be inspected periodically for buildup
of solids. Spray nozzles and liquid inlets must be checked to
see that they are open and distributing the liquid properly.
Corrosion can occur underneath built-up scale. Abrasion is
another ma^or problem in most scrubbers with mechanical or cen-
trifugal shaft actuation devices.
Points of possible corrosion and abrasion must be inspected
frequently. These include throats, orifices, elbows, and any
other high-wear areas. Wear on coatings and metal surfaces
should be repaired as needed. Ductwork, dampers, fans, centri-
fugal pumps, valves, and piping require systematic inspection.
Most fan problems are indicated by a change in vibration.
When a fan is inspected, the casing should be opened and the
wheel and casing washed. The wheel can be checked for erosion,
pitting, and cracking, particularly in weld areas. Many fan
3-28
-------
problems are caused by lack of proper bearing lubrication. If
the fan is equipped with a spray wash system, the piping and
nozzles should be thoroughly inspected. Unusual stresses in the
fan can tear spray piping loose and cause chipping of the fan
wheel.
Centrifugal pumps should be opened periodically and in-
spected. Pump packing should be replaced during each inspection.
The liquid level to the pump gland (packed type) should be checked
once per shift to prevent corrosive or erosive damage of the
shaft, sleeve, and bearing.
The following checklist, based on problems encountered in
scrubber operation, should be followed routinely. Corrections
should be in accordance with the manufacturers' recommended pro-
cedures.
0 Check the scrubber disc in the venturi scrubber to
ensure even distribution across the disc surface.
0 Check erosion and corrosion of all scrubber internal
surfaces. Repair as necessary.
0 Clean and descale all scrubber internal surfaces.
While descaling, exercise care to prevent damage to the
linings.
° Perform maintenance of the hydraulic packing of the
scrubber disc.
0 Check nozzles for buildup or damage. Repair or replace
as necessary.
0 Check for solids buildup in blowdown lines. The lines
may be cleaned without system shutdown, and a flush
connection may be installed to prevent further buildup.
0 Check for corrosion, erosion, and leaks in lines having
protective liners, which may have deteriorated. Replace
liners as required.
3-29
-------
• Check operation of mist eliminator. Formation of
droplets can be caused by excessive gas flow rate,
plugged drains from the moisture eliminator, or con-
densation in the outlet duct.
0 Check pumps for wear, seal water, packing, and smooth
operation.
0 Check dampers and damper linkages for proper position-
ing and wear.
0 Check fans for lubrication, fan bearing coolant, belt
wear and belt tension, and erosion/corrosion of the
impeller.
0 Inspect all interior surfaces and condition of rist
eliminator and sump during major outages.
0 Inspect the exterior for leaks in all process and
control lines, ductwork, and expansion joints.
0 Note the condition of all instruments such as level
probes and density prcbes with regard to solids build-
up. It is impractical and usually impossible to remove
solids buildup from the probes, which often must be
replaced.
0 Perform, a final check for proper operation of density
sensors, pressure drop control, and level elements.
0 In the impingement scrubber, check the liquid level
control and possible solids deposition in the cone
bottom.
Table 3-3 lists general maintenance requirer er.ts fcr venturi
scrubbers based on two ranges of pressure drops, and various
lining materials and gas characteristics. This table should be
useful in the selection cf scrubber liners for lime kilns, smelt
dissolving tanks, combination bark/fossil-fuel boilers, and
various crushed stone processes.
Spare Parts—
The minimum inventory is one of each part for each venturi
scrubber. The inventory for a venturi system is given in Table
3-4.9
3-30
-------
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3-32
-------
Manpower Requirements--
Table 3-5 presents general manpower requirements for main-
tenance involving scaling and plugging for both the wet approach
and liquid injection type venturi scrubbers. The preceding dis-
cussion has given an indication of maintenance items, maintenance
times, and spare parts inventory for a venturi scrubber system.
Table 3-6 completes this picture by presenting the types of per-
sonnel generally required to perform maintenance on various parts
9
of the venturi scrubber system.
Table 3-5. MANPOWER REQUIREMENTS FOR MAINTENANCE^INVOLVING
PLUGGING AND SCALING OF VENTURI SCRUBBER
Type of
venturi
scrubber
Wet
approach
Liquid
injection
Type of problem
Plugging
Mechanical
cleaners
1 man/shift/
mo
1 man/shift/
mo
Cylinder
cleaners
1 man/shift/
mo
1 man/shift/
mo
Scaling
Chemical
cleaning
3 men/shift/
wk
3 men/shift/
wk
Hand
cleaning
1 man/shift/
wk
1 man/shift/
wk
3-33
-------
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3-34
-------
3.4 OPERATION AND MAINTENANCE OF FABRIC FILTERS
3.4.1 Background Information on Fabric Filter Operation
A fabric filter baghouse consists of a large metal box
divided into two chambers of plenums, one for dirty air and one
for clean air. Rows of fabric bags form a partition or interface
between the plenums. A polluted gas stream is ducted into the
dirty-air plenum, where it is distributed evenly to the bags.
The gas passes through the bags, enters the clean-air plenum, and
is exhausted into the atmosphere through a stack.
Upon startup of a baghouse with new bags some stack emis-
sions are usually visible. This is because the bag fabric, which
is the filtering medium, is porous and some of the fine particu-
late passes through the interstices between the fibers. After a
short time, however, a dust cake builds up on the surface of the
bags and becomes the actual filtering medium. The bags then act
as a matrix to support the dust cake.
Buildup of the dust cake is desirable until the system
reaches a certain pressure drop, at which point the bags must be
cleaned. Improper cleaning will cause the pressure drop to
increase; if it becomes high enough, particles of dust may be
forced into the bag filter, causing the bags to become "blinded."
When this happens, air flow is restricted and the bags may have
to be replaced or removed and cleaned to restore proper operating
capacity. In addition to the costs of replacement and cleaning,
high pressure-drop increases the cost of moving air through the
system.
3-35
-------
A typical reverse-air or shaker-type baghouse is shown in
Figure 3-3, and a pulse type baghouse is presented in Figure
3-4. Operation of the various types of cleaning mechanisms is
discussed below.
Shake; Many mechanical shaking methods are in use. Most com-
monly, bags are shaken from the upper fastening. Several com-
binations of horizontal and vertical motion can be used. The
bags -ay all be fastened to a common framework moving horizon-
tally or the frame may have slight additional upward or downward
swing, depending on the linkage holding the framework. The
framework can also be oscillated vertically.
During the shake, the filtering should be stopped. Other-
wise, the dust will work through the cloth, reducing the effi-
ciency and possibly damaging the cloth by internal abrasion. An
effective cleaning method involves a series of alternate flows
and shakes. This motion provides a gentle treatment of the
cloth, and the cleaning is uniform and thorough.
In a typical cycle, the inlet flow to the compartment is
first dar.pered off by a timer. If necessary, the outlet vent is
also closed (Figure 3-5). In the absence of an air lock between
adjacent hoppers, it may be necessary to close a damper to pre-
vent the intrusion of dirty air from hoppers still operating.
There should be zero forward pressure across the fabric during
shaking, since otherwise dust will work through the fabric. The
timer starts the shaker motor, and the bags are shaken. Shaking
continues for about 10 to 50 cycles, each cycle taking about 0.2
3-36
-------
SHAKER
MOTOR
»£nSEI2I33HL£.,- -J££
TLEAN AIR
Figure 3-3. Reverse air or shaker type
3-37
-------
COMPRESSED AIR
Figure 3-4. Pulse jet type,
3-38
-------
to 1 second. Then the timer may start a slight flow of clean
reverse air using an auxiliary blower for 10 to 20 seconds. The
shaking may be repeated, this time during the reverse flow.
Finally, the cleaning is stopped and after a pause to allow the
dust to settle, the inlet and outlet dampers are opened and the
compartment resumes filtering. The entire cleaning cycle may
take from 30 seconds to a few minutes. Some installations do not
return the compartment on line until the next one is ready to be
cleaned, thereby achieving a fairly steady overall flow through
the baghouse system at the expense of some over-capacity.
Reverse Flow; If the dust can be released fairly easily from the
fabric, a low-pressure reversal of the flow may be enough to
loosen the cake without mechanical agitation. To minimize flex-
ural attrition of the fabric, it is supported by a metal grid,
mesh, or rings and is usally kept under some tension. The sup-
port is usually on the clean side of the tube or bag, although
dirty-side support can help to keep the sides of the bag or the
panels far enough apart to allow the cake to fall to the hopper.
Flow reversal is achieved in several ways. In addition to
the standard dampers on each compartment, each one can have its
own reversing fan. A few models have a traveling apparatus that
goes from bag to bag or from panel to panel, blocking off the
primary flow and introducing some air in the reverse direction
with a secondary blower. A simpler method is to take advantage
of suction on the dirty side or of relative pressure on the clean
side without using another blower, as shown in Figure 3-6.
3-39
-------
I~~I
\
FILTERING
TLET S^JT
INLET SHUT
Figure 3-5. Diagran shewing norral operation and shake
cleaning of a fabric filter.10
3-40
-------
Any flow volume reversed through the filter must be refil-
tered. Therefore in addition to taking cloth out of the system
for cleaning, this cleaning method increases the total air flow
in the remainder of the system. The net increase in air/cloth
ratio is normally 10 percent or less.
Plenum Pulse; This method is intended to overcome some of the
difficulties associated with other methods of cleaning. In
plenum pulse equipment a sharp pulse of compressed air is re-
leased in the plenum chamber, giving rise to some combination of
shock, fabric deformation, and flow reversal. The result is the
removal of the dust deposit with only a brief interruption of the
filtering flow. The fabric receives a minimum of flexural wear,
and the filter installation is smaller because the fabric is in
use practically all the time.
The main distinction of pulsed equipment is the brief clean-
ing time, typically around 0.1 second. Because of the very low
ratio of cleaning time to filtering time, pulsed equipment is
especially useful with heavy dust loadings.
Pulsed Jet; This cleaning method is similar to plenum pulse
cleaning. The difference is that in pulse jet cleaning each bag
is individually pulsed, whereas in plenum pulse cleaning the
whole compartment of bags is pulsed by introduction of pulsing
air in the plenum chamber.
3-41
-------
OPTIONAL, TO AVOID
TEMPERATURE CHANGES
« ,
I
PRESSURE
F:
R:
COMPARTMENTS FILTERING
;ARTMENT BEING CLEANED EY DA-PERED
CONTROL FRDM SUCTION SIDE OF SYSTEM
Figure 3-6. Schematic for reverse flow cleaning during
continuous filter operation.1°
3-42
-------
Vibration or Rapping; This method of cleaning is particularly
successful with deposits that adhere relatively loosely to the
bags. The vibration or rapping produces stresses at the fabric-
cake interface, causing release of the dust cake from the fabric.
Sonic Assist: Engineers have attempted to produce agitation
frequencies still higher than those used in vibration or rapping
with ultrasonic and sonic cleaning methods. Although cleaning at
these frequencies slightly improves the preagglomeration of a few
fine dusts, the systems generally have not been very effective in
fabric cleaning.
3.4.2 Fabric Filter Startup and Shutdown Procedures
Preoperation Checks—
The following checks are recommended prior to startup:
0 Test control air lines (hydrostatically).
0 Check air dryers that supply control air to the bag
filters.
0 Check dust removal system.
0 Inspect collapse air fans for alignment and rotation.
0 Check seals at gas inlet, collapse air, and gas outlet
damper.
0 Check baghouse compartments, remove debris.
0 Check filter bags for proper installation and tension.
0 Check and sweep thimble floors clean. Dust buildup on
floor during operation is positive indication of a
broken bag.
0 Calibrate pressure drop recorders and transmitters.
0 Check pressure taps for leakage.
3-43
-------
Startup—
Although operation of a fabric filter system is virtually
completely automatic, startup and shutdown are extremely criti-
cal.
When the new equipment is started for the first time, the
fan should be checked for correct direction of rotation and
speed. The ducting, collector housing, etc., should be checked
fcr leaks. Gas flows and pressures should be checked against the
design specifications. Instruments should then be checked for
correct reading and calibration adjustments made as necessary.
Control mechanisms, and especially all fail-safe devices, should
be checked fcr cperability.
At the first startup of the system, and also whenever new
bags have been installed by the maintenance crew, the bags should
be checked after a few hours of operation for tension, leaks, and
expected pressure differential. Initial temperature changes cr
the cleaning cycle can pull loose or burst a bag. It is wise to
record at least, the basic instrument readings during this initial
startup with new bags for ready reference and comparison during
later startups.
During any startup, transients in the dust generating pro-
cess and surges to the filter house are probable and ought to be
anticipated. Unexpected temperature, pressure, or moisture may
badly damage a new installation. In particular, running almost
any indoor air or combustion gases into a cold filter can cause
condensation on the walls and cloth, leading to blinding and*
3-44
-------
corrosion. Condensation in the filterhouse may void the manufac-
turer's guarantee. It can be prevented by preheating the filter
or the gas.
A typical sequenced startup procedure for a large continu-
ous, automatic, multicompartment fabric filter using either
reverse air, shake, or combination cleaning is summarized as
follows:
1. Check to see that all system monitoring instruments are
reading zero; especially fan motor ammeters and com-
partment pressure manometers.
2. Close all system dampers except tempering air damper
(if used). This includes main compartment isolation
dampers, reverse air dampers (if used), and fan modula-
tion dampers.
3. Start material handling system including any motorized
airlock devices and screw conveyors. Hoppers should be
empty on startup.
4. Sequentially start main fans, allowing each to come to
speed before starting next fan.
5. Start separate reverse air fan if used and allow to
come to speed.
6. Engage fan modulating damper circuit(s).
7. Engage tempering air damper circuit (if used).
8. Slowly open main-compartment isolation dampers. If
dampers are opened too quickly, bags will pop open, ul-
timately resulting in failure.
9. Engage compartment cleaning recircuit.
10. Check normalcy of readings on system monitoring instru-
ments, especially fan motor ammeters and compartment
pressure manometers.
3-45
-------
Shutdown—
1. After process has been stepped and is no longer e-
volving emissions, allow baghouse to track through one
complete cleaning cycle; this will purge system of
process gas and collected dust.
2. Stop main fans.
3. Stop separate reverse air fan, if used.
4. Allow material removal system to operate for 1 hour or
until system is purged of collected material.
3.4.3 Inspection a_nd Maint e n_aji_c_e__pu_r_i n g_ No r m a_l Ope ration
This section presents general maintenance procedures that
car. be applied to fabric filters, on kraft pulp mill power boil-
ers and crushed stone operations. Table 3-7 presents a checklist
cf iters that, require regular inspection.
Plant personnel must learn to recognize the symptoms that
indicate potential problems in the fabric filter, determine the
cause of the problem and remedy it, either by in-plant action
or by contact with the manufacturer or other outside resource.
High pressure drop across the system is a sym.pton for which
there could be many causes, e.g., difficulties with the bag
cleaning mechanism, low compressed-air pressure, weak shaking
action, loose bag-tension, or excessive reentrainment cf dust.
Many other factors can cause excessive pressure drop, and several
options are usually available for corrective action appropriate
to each cause. Thus the ability to locate and correct malfunc-
tioning baghouse components is important and requires a thorough
understanding of the system. A detailed list of troubleshooting
and corrective measures is given in Appendix B-4.
3-46
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Table 3-7. CHECKLIST FOR ROUTINE INSPECTION OF BAGHOUSE'
Component'
Check for:
Shaker mechanism(s)
Bags
Magnehelic gauge or
manometer
Dust removal system
Baghouse structure
(housing, hopper)
Ductwork
Solenoids, pulsing valves
(RP)
Compressed air system
(RP, PP)
Fans
Damper valves (S, PP, RF)
Doors
Baffle plate
Proper operation without
binding; loose or worn bearings,
mountings, drive components;
proper lubrication
Worn, abraded, damaged bags;
condensation on bags; improper
bag tension (S) (RF); loose,
damaged, or improper bag
connections
Steadiness of pressure drop
(should be read daily)
Worn bearings, loose mountings,
deformed parts, worn or loose
drive mechanism, proper lubri-
cation
Loose bolts, cracks in welds;
cracked, chipped, or worn
paint; corrosion
Corrosion, holes, external
damage, loose bolts, cracked
welds, dust buildup
Proper operation (audible com-
pressed air blast)
See above; proper lubrication
of compressor; leaks in headers,
piping
Proper mounting, proper lubri-
cation of compressor; leaks in
headers, piping
Proper operation and synchroni-
zation; leaking cylinders, bad
air connections, proper lubri-
cation, damaged seals
Worn, loose, damaged, or
missing seals; proper tight
closing
Abrasion, excessive wear
RP-reverse pulse; PP-plenum pulse; S-shaker; RF-reverse flow.
3-47
-------
Table 3-8 presents the frequency of failure of basic fabric
filter parts, frequency of inspection and inspection time, and
time required for repairs.
Following is a discussion of major fabric filter components
requiring routine maintenance:
Inlet Dusting—
Common problems such as abrasion, corrosion, sticking or
plugging of dust, and settling must be dealt with on a routine
basis. Abrasion can be reduced by using special materials at
bends in ducting. Corrosion can be minimized by supplying insu-
lation, especially in the long duct runs, which are most sus-
ceptible to moisture condensation. Regular inspection will help
control plugging and dust settling in ducts.
Blast Gate and Flow Control—
Problems with flow control equipment are reported frequent-
ly. The blast gate valve is especially vulnerable and should
be checked periodically and adjusted. Filter compartment inlet
dampers are a high-maintenance item, and spare parts should be
stocked. A bad camper seal can shorten the life of bags in a
shake-type system, and caking bags, if not replaced, can foul
valves on the clean side of the baghouse and cause them to mal-
function. The most popular dampers for compartment isolation are
air cylinder-operated poppets acting vertically (see Figure
3-7). Maintenance on these dampers consists of periodic in-
spection and replacement cf packing and solenoids. Damper fail-
ures can sometimes be detected by observation of a differential
3-48
-------
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3-49
-------
pressure chart. As the dampers open and close, the differential
pressure swings. If a damper fails, the absence of this pressure
swing leaves a "cap" on the differential pressure chart. If a
high differential pressure is signaled, the darr.pers are routinely
checked for proper operation. If not, the operator nust observe
damper operation through the complete cycle directly at the
baghouse.
Fans--
Fans and blowers are reported to present many problems,
particularly those located on the dirty side of the baghouse
where material can accumulate on the vanes and upset the bal-
• ^
ance."" Ccrrcsion and abrasion of fans can also cause problems.
The use cf pressure-style baghouses has been dramatically re-
duced, however, because of the necessity for on-line maintenance
and the dar.ger associated with working on a pressure type inside
bag collector.
Condensation and corrosion in the fan may be alleviated with
10
duct and fan insulation. Most fan housings can be drained, and
the drains should be checked regularly.
Air flew and fan speed should be measured periodically and
belt condition and tension determined; the fan should also be
checked fcr direction of rotation. These checks can be combined
with routine lubrication procedures.
Entrance Baffles—
Any baffles added to improve the distribution of gas to each
compartment and bag should be adjustable. Baffles may cause
3-50
-------
WAFER
.SEAT
PUSH ROD
AIR CYLINDER
OPERATOR
Figure 3-7. Poppet valve.
3-51
-------
problems by accumulating dust or abrading too rapidly.
Hoppers--
Hoppers are a common problem in any fabric filter system.
Dust flow can be facilitated by the use of vibrators and/or
heaters (if they work properly); by lining the hoppers with
antifriction materials; by the use of air-pulsed rubber-lined
hoppers; by placing poke holes in the side of the hoppers; or by
insulation if condensation is a problem.
Trough-type hoppers with integral screw conveyors are the
most comrcr. material handling systems for kraft pulp rill power
boilers and crushed stone operations. Dust storage in baghouse
hoppers is a common industry practice, although this frequently
results in dust bridging and subsequent use of sledgehammers
to break the dust bridge in hoppers. Hopper vibrators can be
used but are expensive and have a tendency to pack the dust and
aggravate the problem if vibration amplitude and frequency are
not correctly selected.
Regular inspection of the hopper (once per shift) is manda-
tory to alleviate problems with the suction-removal system or
those caused by bridging of dust before they become serious.
The screw conveyor flighting inside the hoppers is supported
every 10 to 15 feet by nonlubricated sleeve-type hanger bearings
(see Figure 3-8). Wear on these sleeves and on outboard packed
bearings is the major screw conveyor maintenance problem. The
most common sleeve material is cast iron, although Babbitt, wood,
and various other material have been used.
3-52
-------
SCREW CONVEYOR
FLIGHTING
BAGHOUSE
HOPPER
SIDE WALL
BOLTED FLANGE
"U" - TROUGH
FLANGED DISCHARGE SPOUT TO
GATHER UP SCREW CONVEYOR
OR AIR LOCK DEVICE
Figure 3-8. Typical trough hopper and screw
conveyor arrangement.
3-53
-------
Bag Replacement—
The nost expensive maintenance operation for fabric filter
systems is the complete change of a set of bags. This is accom-
plished by a crew of two to six men, who enter the baghouse and
disconnect each bag at the cell plate and top suspension level
and install a new bag in its place. Two bag attachment tech-
niques are illustrated in Figure 3-9. The purchase price of
replacement bags is given in Table 3-9. The bag life reported by
respondents to the questionnaire survey is given in Table 3-10.
Tension-
The amount of bag tension required for best overall per-
formance varies according to the make of the equipment. Correct
tension is a function of filter dimensions and cleaning mecha-
nism. A bag that is too slack can fold over at the lower cuff,
bridge across, and wear rapidly. Too much tension can damage
the cloth and the fastenings. Shake cleaning in particular seems
to require a unique combination of tension, shake frequency, and
tag properties for best results. In any event, the manufac-
turer's recc-.~er.jat ions should be followed and the tension
checked periodically, especially a few hours after installing a
new bas.
3-54
-------
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3-55
-------
Table 3-9. APPROXIMATE COST OF REPLACEMENT BAGS'
Material
Fiberglass
Nomex
Kermel
Cortex
Teflon0
Acrylic
Cost factor
1.0
2.5
= 6
= 5
= 7
1.2
c _
Based on data from the Mcllvaine Fabric Filter Manual.
-i
As corpared to coated fiberglass, which costs 0.65-1.00 per ft"",
depending on manufacturer and size of installation.
Reflects recent reduction in price.
Table 3-10. BAG LIFE IN KRAFT PULP MILL AND
CRUSHED STONE APPLICATIONS
Application
Kraft pulp rill power boilers
Crushed stone industries
Bag life, months
Range
9-48
Average
15
18
Spare Stock—
It is advisable to stock a complete set of filter elements
in case of an erergency. The spare filter elements should be
clearly labeled and kept well-separated from used filter ele-
ments. Table 3-11 presents a typical list of items that should
be stocked, the approximate quantities, and the approximate
delivery time and costs of parts that must be purchased.
Inspection Frequency--
External maintenance inspection of the filter house is
usually performed daily; the filter elements are typically in-
10
3-56
spected weekly or monthly.
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3-57
-------
Shake Cleaning—
Shaker mechanisms are generally simply supported from each
end by knife-edge bearings set in grooved blocks. A fractional
horsepower motor is used with a yoke linkage to oscillate the
shaker bars (see Figure 3-10). Shaker mechanism maintenance is
centered on the drive arrangement. Periodic lubrication of bear-
ings and checking of alignment are required* The shaking
machinery should also be checked periodically for wear. If the
bags are not being cleaned properly, sometimes a minor adjustment
cf the shake amplitude or frequency can markedly improve cleaning,
If a safe amount of shaking still does not properly clean the
clots, it may be necessary to reduce the filtration velocity for
a few hours.
Reverse-Flow Cleaning—
With this type of cleaning, the only maintenance requirement
is to check the rate of flow (back pressure) and the timing
periodically to keep the residual drag at an economical level.
Shake and Reverse-Flow Clear, in g--
As in sha'-.e cleaning, wherever the bag is flexed the rate cf
wear is apt to be high. Maintenance procedures outlined fcr the
shake and reverse-flow methods also apply here.
Pulse Jet Cleaning-
Since the pulse type apparatus contains almost no moving
parts, hardware maintenance is reduced in comparison with re-
quirements for other cleaning methods. Excessive use of air
cleaning pressure can damage bags by overstretching them. Cor-
3-58
-------
ROCKING MOTION
SHAKER BAR
TENSION NUTS
BAG CAP
CLAMP
Figure 3-10. Typical shaker arrangement,
3-59
-------
rective measures include reducing the frequency of cleaning,
using another type of bag fabric, or reducing the abrasiveness of
the dust.
Instrumentation--
Proper operation of fail-safe mechanisms and automatic con-
trol instrumentation is very important to the safety of the
filter cloth. The location of all sensing instruments should
be checked to see that temperature, air flow, and other operating
conditions are being measured properly. All instruments should
be calibrated after installation and rechecked monthly for sensor
location, leaks (manometer), sticking, and legibility of read-
1 n
out."1" Instrument readings covering one complete operating cycle
should be recorded for future use in routine checks and trouble-
shooting. This record should be posted beside each instrument.
3.4.4 Fa_bric-Filter__pp_e_ra_t_ion and Maintenance Problems Speci f ic
to i-'r a f t _PVl p_ M: l'l ~a"nd 'Cr -_=h'ed' "S-cne' OpVrVt'i'c'n's" "
Kraft Pulp Mill Power Boilers--
With only two U.S. pulp mills using fabric filters to con-
trol their bark-fired power boiler emissions, information on
maintenance problems and practices is very limited. Just one of
tr.e two companies was able to produce some maintenance data.
The primary problem, with operation and maintenance of the
two collectors at Simpson Timber centers on the collection hop-
pers. They have experienced plugging problems due to the very
light nature of the collected dust. Even though the screw con-
veyor can adequately handle the volume of material emptying from
the hoppers, the material tends to briJge. Much of the dust
3-60
-------
consists of submicron NaCl particles. To remedy the situation,
Simpson Timber is trying out a hopper vibrator system that actu-
ates at the end of each cleaning cycle. This system appears to
be relieving the plugging. The Simpson personnel also hope to
develop a good method of sensing that will indicate when the
hoppers begin to plug up. To date they have not devised a relia-
ble mechanism.
Simpson installed a bypass chute on the baghouse hopper for
use during cleanout operations. The screw conveyors could not
handle the large volume of material released when a plugged
hopper was dislodged, so the chute was provided to relieve the
extra load.
Except for some operational problems that arose during the
initial few months of operation, the two baghouses have run
satisfactorily for over 2 years. Routine maintenance is, of
course, performed. Every 3 to 6 months, when the system is off
line, the fan housings and impellers are cleaned by blowing them
out with compressed air. Vibration detectors have been installed
on the fan to warn of impending imbalances. This system allows
maintenance personnel to clean the fans before a scheduled in-
spection if needed.
Bag life is reported to average around 15 months. The usual
cause of bag failure is abrasion against the support cage.
Crushed Stone Processes—
Since the baghouse systems serving crushed-stone processing
plants are small and many have been installed within the past 2
3-61
-------
or 3 years, maintenance data are very sketchy. Plant personnel
rarely keep adequate records. The major problems seem to be
related to the cleaning mechanism and the dust-removal system.
Minor problems occur with other segments of baghouses, but
generally operating experience with fabric filtration in the in-
dustry has been good. Downtime for maintenance usually is not a
serious problem because many of the processing operations do not
run continuously. Baghouse maintenance can be done during non-
operating periods unless extensive repairs are needed.
Even though pulse-jet cleaning mechanisms have few moving
parts when compared with shaker-type systems, they may cause
their share of operating difficulties. At one plant, visited
during this study, major maintenance items were the blow-down
valves and diaphragms associated with the pulse-jet system.
These components failed at the rate of almost one a week. Prob-
lems were also encountered with the air compressor bearings and
drive belts. The bearings had to be replaced after only about 4
months of use, and the drive belts also wore quickly. In addi-
tion, the pulse-jet system was regulated by solid-state circuit
cards, which have sometimes failed and needed replacement.
Another plant, reported similar difficulties with their
pulse-jet system. Diaphragms have failed, and the cycling mech-
anism has malfunctioned on several occasions.
Problems with dust removal/conveying systems have plagued
several of the baghouse installations surveyed. Failure of
rotary air locks to remove collected dust from hoppers fast
3-62
-------
enough is a common complaint. This sometimes results in the
material packing in the hopper as it backs up, bridging the
opening just above the airlock. It then becomes necessary to rap
the outside of the hopper to dislodge the dust. Screw conveyors
also are reported as a fairly major maintenance item. At least
two installations reported broken shafts resulting from over-
loading the screw. Maintenance personnel at one plant experi-
enced repeated problems with broken shaft pins. The shaft pin
connects the motor shaft to the screw shaft and will shear when
the screw overloads while the motor still tries to turn at a
constant speed. This problem was most pronounced upon startup on
the morning after collected dust had remained in the hopper and
screw overnight. The baghouse is now run for a short period each
night before shutdown to clear the screw of any remaining ma-
terial.
Other baghouse installations surveyed reported no particular
maintenance problems with their dust conveying systems other than
routine maintenance, e.g., repacking bearings, cleaning out
solidified material. One of the maintenance personnel recom-
mended that baghouse manufacturers provide access panels beneath
the screw conveyor for easier cleanout. With present designs,
someone must enter the baghouse hopper with a light and dig out
the screw from above.
This employee also suggested that baghouses be equipped with
some sort of warning device, either a flashing light or rotating
flag, to indicate when the screw shaft breaks from the motor
3-63
-------
shaft. From a distance, it is difficult to tell that the screw
has stopped turning because the drive motor continues to operate.
The ID fans used with the pulse-jet type baghouse are a
relatively low maintenance item. One plant reported that they
check the fan impeller housing every 3 to 6 months, depending on
the ambient weather conditions. Because dust buildup on the im-
peller blades causes an imbalance and increases bearing wear, it
is advantageous to keep the fans reasonably clean. Erosion of
the blades is not a problem because the rock dust is not highly
abrasive.
Bag life in fabric filters at crushed stone plants may range
frcm 9 r-.cnt.hs to 4 years or more. The average bag life in the
industry appears to be about 1-1/2 to 2 years. The usual cause
cf bag failure is abrasion against the cage supports (for pulse-
type collectors). Excessive moisture in the baghouse inlet air
during wet ambient conditions may pose a problem of bag caking.
In most cases, however, dry dust emissions are minimized curing
wet weather and the ccllectors can usually be turned off.
3-64
-------
REFERENCES - SECTION 3
1. Bump, R.L. Precipitator Design for LowrOdor Boilers Offer
Special Problems. Pulp and Paper. October 1976.
2. Henderson, J.S. Precipitator Survey on Non-Contact Recovery
Boilers, TAPPI Volume 58, May 1975.
3. Bump, R.L. Electrostatic Precipitator Maintenance Survey.
TC-1 Committee of the Air Pollution Control Association.
1974.
4. Environmental Pollution Control Pulp and Paper Industry,
Part I; Air. EPA-625/7-76-001, Chapter 16, p. 16-17.
October 1976.
5. Cross, Frank L. and Howard E. Hesketh. Handbook for the
Operation and Maintenance of Air Pollution Control Equipment,
Pages 56-57, Chapter III. Technomic Publishing Company,
Westport, Connecticut. 1975.
6. Reference 2, p. 48.
7. Stern, Arthur C. Air Pollution. Third Edition, Volume IV,
Ch. 3, p. 130. 1977.
8. Jones H.R. Fine Dust and Particulate Removal. p. 164,
Noyes Data Corporation. Park Ridge, N.J., 1972.
9. Industrial Air Pollution Control. Chapter 7. PEDCo Envi-
ronmental, Inc., prepared for U.S. Environmental Protection
Agency, Environmental Research Information Center EPA-625/6-
78-004, June 1978.
10. Billings, C.E. and J. Wilder. Handbook of Fabric Filtration
Technology, Volume I. Prepared by GCA Corporation for the
National Air Pollution Control Administration, Contract No.
CPA-22-69-38, December 1970.
3-65
-------
SECTION 4
'FRACTIONAL EFFICIENCY RELATIONSHIPS
4.1 INTRODUCTION
This section evaluates the total mass and fractional effi-
ciency capabilities of precipitators, scrubbers, and fabric
filters on Kraft pulp mill and crushed stone industry processes.
Unfortunately, the availability of fractional efficiency test
data on these processes is very limited, and numerous contacts
with users and manufacturers have yielded none; however, fraction-
al efficiency test data for electrostatic precipitators (ESP's)
on kraft pulp mill recovery boilers and a mobile fabric filter
test on a lime kiln were obtained from the literature.
Results from computer models for ESP's and venturi scrubbers
are presented, which predict penetration as a function of particle
size. An appropriate predictive model is not available for use
with fabric filters.
4.1.1 Limitation of Current Data
Only in the past 4 or 5 years has particle size distribution
been measured and recorded with any regularity by control equip-
ment manufacturers, independent testing companies, and consultants;
and because of operator error and the inherent technical limita-
tions of some particle-sizing instruments, reliable data are still
not readily available. Meaningful evaluation of fine particulate
emissions will require development of a reliable and consistent
fine-particle measuring technique that can be applied widely. A
4-1
-------
broadly applicable technique for compliance monitoring of fine-
particle sources would have the added advantage of enabling the
collection of valuable data concerning the subject processes in
this report under different operating conditions.
4.1.2 S_unna_ry_of_ Inl_e_t _Particle_ Si ze Distribution Data Used for
Prec:p i ta tor
Mode 1 s
Table 4-1 suirxnarizes the inlet particle size distribution
data used in the precipitator and scrubber model predictions.
These particle size distributions are based on data from various
literature sources. (See Table 4-1.).
Table 4-1. SUV-MARY OF INLET FAPTICLE SIZE DISTRIBUTION'
DATA USED IN ESP AND SCR'oBbER PREDICTION MODELS
Process
Kraft Pulp Mill
Conventional recovery
boilers
Low-odor recovery boilers
Bark/fossil fuel-fired
boi ler s
c ^ u ~ ~ e n i -1 e kilns
Crushed S t c r. e Industry
Jaw crushers
Conveyers
x , um
1.4-1.9
1.5
5-15
20
200
10
cg, pm
3.0-2.04
2.5
2.5-4.0
4 5
8.7
4
Reference
1,2
3
4
5
6
6
4-2
-------
4.2 PROCEDURE FOR DETERMINING FRACTIONAL EFFICIENCY PERFORMANCE
4.2.1 Electrostatic Precipitator Computer Model
Two computer models used to predict precipitator performance
were described earlier (Section 2.3.2). In the first model
reentrainment losses are assumed to be negligible. Because
reentrainment does occur with precipitators installed on bark and
combination-fired boilers, another theoretical model is presented
in Section 2.3.2 to account for it. This section presents selec-
ted results of applying the model to predict the fractional
efficiency of an electrostatic precipitator.
Model Application to Conventional and Low-odor Recovery Furnaces—
The first theoretical model described in Section 2.3.2
(specifically equation 17) can be used to compute the outlet
particle size distribution and fractional efficiencies for given
values of inlet particle size distribution and required overall
mass collection efficiency. Part of the program logic involves
an iterative procedure in which the parameter, K, is adjusted
until the computed efficiency is close to the desired value
within specific limits. Once an appropriate value of K has been
determined, the fractional penetrations can be estimated by use
of equation 15.
Figure 4-1 shows particle penetration as a function of
particle diameter for precipitators applied to conventional and
low-odor recovery furnaces. The minimum collection efficiency
occurs in a particle size range of 0.2 to 0.6 ym. The reason is
that the electrical mobility of particles is at a minimum in this
4-3
-------
u
O
c en
0 o»
•H CJ
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C
0) >,
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1- >
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4-4
-------
size range. Note that the field charging equation is important
for particles greater than 0.5 ym. The equation predicts parti-
cle mobilities that increase with increasing particle size. The
diffusion charging theory predominates with respect to collection
of particles smaller than 0.5 ym. This theory predicts that the
particle mobility decreases with increasing particle size, and
minimum mobility occurs in the size range of 0.4 to 0.7 ym.
Thus the range of 0.2 to 0.6 ym presents the most difficult size
range for operation of electrostatic precipitators.
Figures 4-2 and 4-3 are similar plots of penetration as a
function of particle size for selected overall mass collection
efficiency levels based on two inlet distributions having coarser
particles. Although the range for minimum collection is still
0.2 to 0.6 ym, the absolute fractional efficiencies at the same
overall mass collection efficiency depend very strongly on the
inlet size distribution. For example, at 0.3 ym and at an over-
all mass collection efficiency of 99.9 percent, Figure 4-1
indicates a fractional efficiency of 98.9 percent, whereas Figures
4-2 and 4-3 show fractional efficiencies of 97.8 and 97.0 percent,
respectively.
One set of fractional efficiency test data for an ESP on a
kraft pulp mill recovery boiler was obtained from a report by
Gooch, et. al. of Southern Research Institute (SRI). These
results show an average overall mass collection efficiency of
99.96 percent with a minimum average collection efficiency of
99.92 in the size range of 0.15 to 1 ym. The mass median diameter
4-5
-------
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4-7
-------
of the particulate entering the collector was approximately 1 urn.
Figure 4-4 presents fractional efficiency results obtained by
SRI using an electric aerosol analyzer and inertial impactors.
Collection efficiencies are uniformly high between 0.1 and 10 um,
but decline slightly below 0.1 um, contrary to what would be
expected by theory. The SRI model underpredicts efficiency below
1 um, presumably due to the method in which particulate space
Q
charge is calculated within the model. Above 1 um, the SRI
model cverpredicts efficiencies because of reentrainment effects
g
en the larger particles. Research-Cottrell's model would show
results similar to the SRI model if applied to the same situation.
(See Figure 4-1, at 99.9 percent efficiency, for example.)
Another set was obtained from the Fine Particle Emission
Information System (FPEIS) for an ESP on a kraft pulp mill
9
recovery boiler. The results from a number of these tests are
presented in Figure 4-5. Overall mass efficiencies ranged from
95.89 to 99.34 percent. As shown in Figure 4-5, a minimum
efficiency occurs in the 0.2 to 0.4 um range as would be predicted
by theory. Another minimum efficiency is beginning at about 4
„.-, the upper reported limit of the i-pactor test data, presum-
ably due to agglomeration of smaller particles and subsequent
reentrainment from rapping. Figure 4-6 shows approximate frac-
tional efficiency curves for two normal ESP's and one with severe
reentrainment. The minimum point in collection efficiency for
the normal units occurs at 8-10 um, while the unit with reen-
trainment problems shows no improvement in efficiency out to a
particle size of 100 um. These curves do not extend far enough
4-8
-------
0.01
0.05
0.1
0.2
0.5
1
2
20
30
40
.01
SRI MATHEMATICAL MODEL
• E.A.A.
• IMP.
0.1
PARTICLE SIZE.pm
i:
99.99
99.9
99.6
m
99 2
98 5
m
95 3
90 •""
v
80
70
60
10
Figure 4-4. Measured and theoretically calculated fractional
efficiency of an ESP on a Kraft Pulp Mill recovery boiler.8
4-9
-------
FINE PARTICLE EMISSIONS INFORMATION SYSTEM
TEST SERIES 18
COMPOSITE AVERAGE
FOR SU8SERIES 1 4 2
15 & 16
22 & 23
37 & 38
10
9
8
7
6
5
.0
.9
.8
.7
.6
.5
.4
.3
.2
T
KEY
D
6
O
SUESERIES 1 & 2
S'JBSERIES 15 4 16
SL'BSERIES 22 & 23
SUBSERIES 37 & 38
PARTICLE SIZE, urn
Figure 4-5. Penetration as a function of particle size
an ESP on a kraft pulp mill recovery boiler.
for
4-10
-------
.9
.7
.5
o 99
97
95
90
70
I I
J I
I I
1.0
10
PARTICLE SIZE,
SEVERE
REENTRAINMENT
100
Figure 4-6. Fractional collection efficiency of
precipitator collecting particulate from pulp mill
recovery boiler. ^
4-11
-------
below 1 ;.m to determine whether another minimum in collection
efficiency occurs in the 0.2 to 0.4 um range. The minimum in
collection efficiency at 8 to 10 urn for the normal units could be
the result of normal reentrainment effects.
.Model Application to Bark Combination Bark/Fossil Fuel-fired
Sciiers--
The theoretical model for design of precipitators for bark/
fossil fuel-fired boilers (incorporating a correction factor for
reer.trair.rer.t losses) is described in Section 2.3.2. That
formulation is used here to develop penetration/particle size
correlations.
Equation 37 shows that a numerical integration procedure is
needed to evaluate overall mass collection efficiency. Input
parameters consist of the following:
i) Known inlet particle size distribution, i.e., f (d) ,
represented by a log normal plot and the values cf x
and c to cover the entire particle size range.
11) The value of the parameter K (equation 13) in the
absence of reentrainment loss. This can be determined
by using the iterative procedure described in Section
4.2.1.
lii) The percent material fraction that is reer.trained over
the given number of mechanical sections. In equation
36 these variables are r and n.
With values for the above quantities and equation 4 one can
estimate percent penetration as a function of particle size.
4-12
-------
Precipitators installed on bark-fired and combination-fired
boilers usually have four mechanical sections.
Figure 4-7 shows estimated collection performance as a func-
tion of particle size at four different levels of particle re-
entrainment r. Note that the minimum collection is still in the
range of 0.2 to 0.4 ym. As r increases, however, the first r-
term in equation 36 is predominant relative to the second term,
which includes the particle-size-dependent parameter, w,. As
an illustrative example, assume that r is 0.4, i.e. 40 percent
reentrainment, and n is 4, i.e. four mechanical sections. In
4
this case the penetrations approach r , which is 2.56 percent,
irrespective of increasing particle size. Figures 4-8 and 4-9
are similar plots with different assumed inlet distributions.
Experimental field test data are needed to support the validity
of this formulation.
4.2.2 Wet Scrubber Computer Model
Design Equations and Assumptions—
Venturi scrubbers are well described in the available litera-
ture. ' The particle collection process depends mainly upon
the acceleration of the gas to provide impaction and intimate con-
tact between the particles and fine liquid drDplets generated as
a result of gas atomization. The other factor that influences
the effectiveness of the venturi scrubber is condensation. If the
gas in the reduced-pressure region in the throat is fully saturated,
condensation will occur on the particles in the higher-pressure
region of the diffuser; this phenomenon, known as heterogeneous
nucleation, helps particle growth and also causes agglomeration,
4-13
-------
3'.
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4-16
-------
which tends to enhance collection. Detailed particle collection
mechanisms in the venturi scrubber have been investigated by many
researchers.13'14'15
The venturi model used in this study is based on inertial
impaction. It is assumed that the particles do not grow during
the collection process as a result of heterogeneous nucleation
and condensation effects. The general form of the expression for
collection efficiency with particle size i can be written as:
Ei = 1 - exp(-K(L/G)H'i) (Eq. 40)
where E. = Removal efficiency, fractional
K = Impaction correlational parameter (system
parameter)
L/G = Outlet liquid-to-gas ratio, gal/1000 acf
y. = Inertial impaction parameter of particle size
1 grade, i
Available experimental data have been used to develop a
correlation for inlet throat velocity, V, in ft/s, based on AP
(in. H.O) and outlet L/G measurements.
AP 1/2
V = ^ (Eq. 41)
c 5.23 X 10 (L/G + 105)
Knowing the inlet throat velocity and measured outlet L/G,
one can calculate the droplet diameter from a modified form of an
equation developed by Nukiyama and Tanasawa.
1.5
n = + 1>41 (L/Q)
t
The system parameter, K, is determined by an iterative
procedure based on comparison of the actual measured overall mass
4-17
-------
collection efficiency and that calculated from summing the in-
dividual fractional efficiencies. For a given particle size the
inertial impaction parameter is defined below:
0.85(C) (D ) (D )2 V
where C = Cunningham correction coefficient
c = Particle specific gravity, g/cm
D = Particle diameter, um
P
4
= Dynamic gas viscosity, poise x 10
D_ = Droplet diameter, ..n
2\ D
and C = 1 f ~ 1.23 + 0.41 exp(-0.44 -&) (Eq. 44)
P
The value of K is modified during the course of iteration to
yield a closer match between measured and calculated overall mass
collection efficiencies for given input values of L/G and IP.
When the "optimum" value of K has been found, it is inserted into
the above equations to generate the outlet particle size distri-
bution and finally the fractional penetration for the various
particle sizes. This is really an averaged system parameter,
since it is not a function of any specific particle size.
Penetration as a Function of Particle Size--
Figure 4-10 shows penetration as a function of particle size
for venturi scrubbers collecting sludge lime kiln dust. Typical
ranges of operating parameters were taken from general literature,
and the inlet particle size distribution was derived from data
presented by Cheremi sinof f , who does not mention how much soda
fume (Na-0) is present. Since these particles are generally
4-18
-------
s
100
90
80
70
60
50
40
30
20
10
9
B
7
6
5
4
3
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
I I I 1 I 1 I
x • 20, • • 4.5
(INLET)
I 1 i 1 1 I 1 r
1 i I I i i I
j 1,1 l
i ill]
^^ CO O1* ^^
o oo —
o
CM
OOOOO
PARTICLE SIZE,
Figure 4-10.
Predicted penetration for venturi scrubbers
on sludge liroe-kilns, L/G = 15.
4-19
-------
considerably less than 1 um and constitute up to 2 percent of a
typical unburned lime, they can significantly raise scrubber
pressure requirements. Pressure drops substantially above 20 in.
H20 would be required to achieve mass collection efficiencies
above 99 percent. In this case the use of an x and c to specify
the inlet particle size distribution is sorr.ewhat deceiving be-
cause it does not adequately reflect the preponderance of fine
Computer-predicted penetrations for venturi scrubbers on jaw
crushers and conveyors are presented in Figures 4-11 and 4-12.
The or.ly available fractional efficiency test data for ven-
turi scrubbers on any of the subject processes cor.es from unpub-
lished impactor test data for a venturi scrubber operating on a
salt laden bark/oil fired boiler. Although the inlet and outlet
tests were taken on different days, the boiler conditions were
sir.iiar for both tests, and so an approximate fractional efficiency
curve was prepared, and is presented in Figure 4-13. Overall mass
efficiency for these test data is approximately 63 percent.. The
characteristic snarp increase in penetration as particle size
decreases is evident in this cruve.
Overall mass and some fractional efficiency pilot tests on
hogged fuel boilers have been conducted with novel type electro-
static scrubbers. The TRW charged droplet scrubber, the Ceilcote
ionized wet scrubber, the APS electrostatic scrubber, and the
University of Washington electrostatic scrubber are the novel
type electrostatic scrubbers which have been tested. Table 4-2
ccrpares characteristics and performance of these electrostatic
4-20
-------
PARTICLE SIZE.
Figure 4-11.
Predicted penetration for venturi scrubbers
on jaw crushers; L/G = 5.
4-21
-------
o o
<"s<
PMTJCU SIZE. w*
Figure
4-12.
on
Predicted penetration
convevors and screens
for venturi
L/G = 5.
scrubbers
4-22
-------
100
50
o
t-^
I—
oc
10
OVERALL MASS EFFICIENCY = 68%
"x = 0.63 og = 7.14
J I I I I I I
.1
.5 1.0
PARTICLE SIZE,
10
Figure 4-13. Approximate penetration as a function of
particule size for a venturi scrubber operating
on a salt-laden bark/oil fired boiler.
4-23
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4-24
-------
scrubbers with a conventional venturi scrubber operating on a
salt-laden bark/oil fired boiler at a 20 in. H-0 pressure drop.
4.3 EFFICIENCY RELATIONSHIPS FOR FABRIC FILTERS
It would be desirable to characterize baghouse collection
efficiency as a function of particle size for the processes dis-
cussed. However, manufacturers seldom base baghouse guarantees
on fractional collection efficiency. They prefer overall collec-
tion efficiency or outlet concentration guarantees based on their
past experience with the same or a similar type of process.
There is little incentive for manufacturers to relate baghouse
performance to particle size on existing systems and as a result,
this information is practically nonexistent. Even though frac-
tional efficiency data for baghouses in the kraft pulp and
crushed stone industries are lacking, there is adequate evidence
in the literature from other industries to document the fact that
fabric filters preserve good fractional efficiency of filtration
in the submicron particle size range. In other words, unlike
scrubbers and ESP's fabric filters preserve high collection
efficiency for the small particle size ranges without any modifi-
cation to their design or operation.
It must be pointed out, however, that there are several
subtleties reported in the literature regarding the actual shape
of the fractional efficiency curve for a fabric filter. There
are reports suggesting that the collection efficiency of a fabric
filter tends to drop somewhat in the submicron particle size
range particularly in the 0.2 to 0.4 pm region. This behavior is
4-25
-------
also common to scrubbers and ESP's except that in their case the
drop in efficiency is more pronounced. in one recent report,
it is postulated that mechanisms during high velocity filtration
can actually lead to decreased filtration efficiency on larger
particles. The three mechanisms are as follows:
1. Straight Through Penetration^. Immediately after clean-
ing, many particles collect upon the exposed fibers.
Soon, however, a continuous dust deposit forms on the
fabric surface and particles collect upon previously
deposited dust. Particles not collected by the filter,
but which pass through without stopping penetrate the
filter by the "straight through" mechanism.
2. 2e_eP_aJL£.: Once a particle lands on or in the fabric, it
need not necessarily remain at its point of initial
impact. As the dust deposit builds up, pressure drop
can increase to several times its initial value. Mean-
while, the fabric substrate may stretch, allowing some
-reviously collected particles to work through. Filter
behavior of this sort is "seepage."
3. ?_i_nhole_ F_lu_a: Srall diar.eter pinholes occur at the
s-rface cf dust deposit on woven fabrics. Similar
holes on a needle-punched felt may correspond to the
places where needles penetrated the cloth during its
manufacture. A plug of deposited particles may dis-
lodge from the dust deposit and pass through the fabric
all at once as the supporting fibers move and stretch
4-26
-------
beneath it, leaving behind such a pinhole. Particles
which pass through the filter in this way do so by a
"pinhole plug" mechanism.
Therefore, particles can pass through the filter by the
"straight through" mechanism without being stopped,
whereas previously collected particles can make their
way through by the "seepage" and-"pinhole plug" mecha-
nisms. In addition, the fraction of the total fabric
filter penetration represented by "pinhole plugs" can
be as high as 70 percent. This could account for the
presence of large particles on the outlet side of
baghouses in amounts not expected by considering clas-
sical filtration theory.
One set of fractional efficiency test data was obtained from
the FPEIS for U.S. EPA's mobile fabric filter applied to a lime
kiln at a kraft pulp mill. Results from a number of these tests
are presented in Figure 4-14. Overall mass efficiencies were
above 99.9 percent. Penetration in the 0.4 to 10 ym range is
less than 1 percent. A minimum efficiency occurs at approxi-
mately 0.8 to 1.0 ym.
None of the plants contacted during this study could provide
fractional efficiency information. In fact, many of the crushed
stone installations did not ever have total mass efficiency data
since their control systems had never been tested for verifica-
tion of the manufacturer's efficiency claim. Therefore, the
subsequent discussion is limited to a review of reported overall
efficiencies in the two subject industries.
4-27
-------
KIT
5.C
0.5
§
*f
S
0.10
0.05
0.0'
0.1
D
C
TEST SERIES 105.
UST SERIES 105.
TEST SERIES 106,
TEST SERIES 106.
SUBSERIES 5 » 6
SUSSERIES 21 1 22
SUBSERIES 13 I 14
SJBSER1ES 21 t 22
TEST SERIES 107. SuBSERIES 516
T
FINE PARTICLE E*!SSi>
STSTE"
AVERAGE
TEST SERIES 105
Si'SSERIES 5 I 6. 21 I 22
T£S* SERIES 106
SLBSEKIES
US' SERIES 107
t 14. 21 I 22
SuBSERIES 546
0.5 1.0
5.0 10
PARTICLE DIAMETER,
50
IOC
Figure 4-14. Penetration as a function of particle size
for a mobile fabric filter on a kraft pulp null lime kiln.
4-28
-------
4.3.1 Bark-Fired Power Boilers in the Kraft Pulp Industry
Neither of the two pulp mills contacted had fractional
efficiency information for the baghouses servicing their bark-
fired boilers. The overall mass efficiency, however, is reported
to range from 90 to 95 percent in one unit with outlet loadings
of 0.01 and 0.04 gr/scf adjusted to 12 percent CCu. This lower
than normally expected efficiency for a fabric filter assumes
that a certain number of the bags are always leaking or broken.
The other facility reports an efficiency of 99.0 percent. Because
the logs used in the former pulp mill are stored in seawater, the
particulate emissions from the bark boilers contain a high per-
centage of sodium chloride. These particles are generally in the
submicron range and contribute to atmospheric haze. The plant
personnel at this firm reported that the primary reason for
installing baghouses on both boilers was to reduce the emission
of submicron particles. After installation of the baghouses, the
boilers have been effectively meeting the applicable opacity
regulations, suggesting that these fabric filters are efficient
collectors for submicron particles. The reported overall effi-
ciencies of 99 percent fromthe latter non-salt facility or higher
are typical of fabric filters in general, and this same level of
efficiency could be extrapolated to other bark boilers.
4.3.2 Crushed Stone Operations
Again, very little data are available on actual efficiencies
of baghouses servicing stone crushing operations. There have
been no known fractional efficiency tests reported in the litera-
4-29
-------
ture. The majority of emission tests report only the effective-
ness of the dust control system regarding compliance with local
emission regulations. Table 4-3 presents the overall mass
efficiency data for baghouses installed at a crushed stone plant,
These data are taken from errission test results reported in the
1iterature.
Table 4-4 presents data on overall collection efficiencies
c: fabric filters at plants contacted during the course of this
study. It also includes the efficiencies of baghouses at two
crushed stone plants visisted during the study. These are in-
dicated as Plants 5 and 6.
4-30
-------
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4-31
-------
Table 4-4. BAGHOUSE PARTICULATE EFFICIENCIES - SURVEY DATA
Plant number
1
2
3
Dust source
NA
Five stone crushers
Ten pickup points including
crushers, impactors, belts, etc.
Overall mass
efficiency, %
99.0
99.88
99.0
Stone dryer
Stone crushing, screening, and
conveying
Fine grinding
Secondary crushing and screening
Storaae silos
= 100.0
99 +
99.0
99.9
99.8-99.9
NA = I.-.f crrat ion not available.
The above data indicate that baghouses servicing stor.e-
crushir.g operations have an average overall efficiency in excess
of 99 percent by weight. This high efficiency is in part a
result of the coarse nature of dust emtted fron stone crushing
coeraticns.
4-32
-------
REFERENCES SECTION 4
1. Gooch, J.P. et al. Particulate Collection Efficiency Mea-
surements of an Electrostatic Precipitator Installed on a
Paper Mill Recovery Boiler, Southern Research Institute.
May 1971, PB 255-297.
2. Oglesby, S. and G.B. Nichols. A Manual of Electrostatic
Precipitator Technology, Part II: Application Area, p. 345
(1970) PB 196-381.
3. Paul, John E. Application of ESP's for Control of Fumes
from Low Odor Pulp Mill Recovery Boilers, JAPCA Vol. 25, No.
2. 1975.
4. Cupp, Stanley, J. Operating Experience with a Boiler Firing
Salt Water Borne Hogged Fuel. Crown Zellerbach Corp., Port
Townsend Division, Port Townsend, Washington, 1978.
5. Cheremisinoff, P.N. and R.A. Young. Air Pollution Control
and Design Handbook, Marcel Dekker, Inc., New York, New
York. 1977. p. 841.
6. Vandegrift, A.E. et al. Particulate Pollutant Systems
Study - Vol. 3: Handbook of Emission Properties, Midwest
Research Institute. May 1971, PB 203-522.
7. Feldman, P.L. Effects of Particle Size Distribution on the
Performance of Electrostatic Precipitators. Presented at
the 68th annual meeting of APCA, No. 74-02.3, (June 15-20,
1975).
8. Gooch, J.P. et al. Particulate Collection Efficiency Mea-
surements on an Electrostatic Precipitator Installed on a
Paper Mill Recovery Boiler, Southern Research Institute,
EPA-600/2-76-141, May 1976.
9. Riggenbach, J.D. et al. Fine Particle Emissions Information
System Series Report, Test Series No. 13, Environmental
Science, Inc. 1973.
10. Nichols, Grady B. Particulate Emission Control from Pulp
Mill Recovery Boilers with Electrostatic Precipitators in
IEEE Transactions on Industry Applications, Vol. 1A-13,
No. 1, January/February 1977.
4-33
-------
11. The Mcllvaine Scrubber Manual, Vol. I, The Mcllvaine Co.,
1974.
12. Wet Scrubber Systems Study, Vol. I, Scrubber Handbook,
A.P.T., Inc., PB 213 016 (July 1972).
13. Johnstone, H.F., R.B. Field, and M.C. Tassler. Industrial
Engineering Chemical, Vol. 46 1601 (1954).
14. Johnstor.e, H.F. and F.O. Eckman. Industrial Engineering
Chemical, Vol. 43, 1358 (1951).
15. Nukiyama, S. and Y. Tanasawa. Trans. Soc. Mech. Engrs.,
Japan, Vol. 5 62-68 (1939).
16. Rydholm, S.A. Pulping Processes, Inter science Publishers,
New York, New York. 1965. p. 802.
17. The Mcllvaine Scrubber Manual, Vol. II, Chapter IX, p. 32.2.
IS. Mcllvair.e, R.W. Fine Farticulate Scrubbing New Problems and
Solutions in Second EPA Fine Particle Scrubber Symposium.
New Orleans La., May 2-3, 1977. EPA-600/2-77-193." Ccrpiled
by R. Parker and S. Calvert.
19. Private corj-.unication with Paoer and pulp company, Au crust
1978.
20. Leith, D. et al. High Velocity High Efficiency Aerosol
Filtration, EPA-600/2-76-020. January 1976.
21. Fine Particle Emissions Information System Series Report,
Test Series Nos. 105, 106, and 107, Selected runs; Monsanto
research Corp., EPA Contract No. 63-02-1816, 1975.
4-34
-------
SECTION 5
SUMMARY AND CONCLUSIONS
This report reviews the use of conventional control devices
(ESP's, scrubbers, and fabric filters) for limiting particulate
emissions from kraft pulp mill and crushed stone industry proc-
esses. Principal areas of study are: important design param-
eters, operation and maintenance procedures, and fractional
efficiency capability of each control device. The following
sections summarize the report and present conclusions drawn from
each area of study.
5.1 DESIGN PARAMETERS
In evaluation of alternative methods of particulate control
for each of the subject processes, it became clear that one type
of control device usually predominated. For example, nearly all
kraft recovery boilers are controlled with ESP's. Some scrubbers
have been installed on U.S. kraft recovery boilers, but only as
retrofit controls. Smelt dissolving tanks are controlled by low-
energy scrubbers, and lime kilns, almost exclusively by venturi
scrubbers. One U.S. mill has retrofitted an ESP to serve three
existing kilns.
Combination bark/fossil-fuel boilers are an exception, in
that ESP's, scrubbers, and fabric filters all have been used for
control of particulate emissions. Use of these devices is
5-1
-------
relatively new and until recently bark/fossil-fuel boilers were
controlled with mechanical collectors. Where hogged fuel con-
tains salt accumulated during transport by sea, the fine salt
particles can cause excessive opacity of the plurr.e. In these
circumstances only venturi scrubbers and fabric filters have been
used for particulate control. Presumably since ESP manufacturers
will guarantee meeting transmissometer opacity but not visible
opacity regulations when salt-laden fuel is burned, no ESP's have
been installed on bark boilers with a salt emission problem.
In the crushed stone industry, fabric filters are preferred
for control at point sources. Wet suppression is also used alor.e
and in conjunction with fabric filters, and low-energy venturi
scrubbers have been used on conveying and crushing processes.
High-energy venturi scrubbers would provide better performance,
but none are presently in use.
General advantages and disadvantages of applying ESP's,
scrubbers, and fabric filters to the subject processes are
presented in Tables 5-1 through 5-3.
£S?' s
The use of ESP's on conventional and lew-odor recovery
boilers and on bark/fossi1-fuel boilers was evaluated. Differ-
ences in ESP design for use on conventional and low-odor boilers
are attributed to differences in temperature, bulk density,
tenacity of dust, and SO- content. The particle size distribu-
tions in effluents of conventional and low-odor boilers are
5-2
-------
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-------
g
similar, and resistivity of both process dusts is around 10 ohm-
err..
Because of higher SO. levels, the dust from low-odor boilers
adheres very tenaciously to the collecting plates and necessi-
tates heavy rapping. This vigorous rapping, combined with the
lover bulk density of the low-odor boiler dust, leads to higher
re-entrain.ment of particles. Moreover, the higher viscosity of
the low-odor boiler dust (due to its temperature) increases the
crag force on the dust particles. Because of these factors, the
SCA requirere.it for an ESP on a low-odor boiler is about 15
percent greater than with a conventional boiler. Additional test
data are reeded to verify this finding. In any event the design
SCA's for high-efficiency (99.5% or greater) ESP's on kraft pulp
mill recovery boilers should be in the range of 380 to 450 ft /
1000 acfm.
7 9
The low resistivity (10 - 10 ohm-cm) of the bark ash
causes serious problems of re-entrainment for ESF's. A correc-
tion factor that assu-es a constant rate of re-ent ra inm.ent in each
mechanical section of the ESP is applied to the modified Deutsch
design equation fcr SCA, and accounts for this high rate of re-
er. train men t. Thus in a high-efficiency ESP application (99.5%
for example) on a bark/fcssil-fuel boiler, SCA requirements would
range from, approximately 380 to 500 ft /1000 acfm assuming 40 to
50 percent re-entra inrnent of the bark ash. Where firing of bark
would cause a salt emissions problerr, the SCA requirements would
be higher because of the smaller size of salt particles.
5-6
-------
Wet Scrubbers
Emphasis in this study is on venturi scrubbers because their
particulate removal efficiencies are higher than those of im-
pingement type scrubbers, mesh demisters or packed towers. Newer
lime kiln installations use venturi instead of impingement scrub-
bers. Most smelt dissolving tanks use mesh demisters and/or
packed towers to collect particulate from exhaust gases. Venturi
scrubbers are used successfully on bark/fossil-fuel boilers.
Design requirements for venturi scrubbers for kraft pulp
mill applications (lime kilns and bark/fossil-fuel boilers)
include pressure drops in the range of 10 to 20 in. H-0 and L/G's
of 10 to 15 gal/1000 acf to achieve collection efficiencies of 90
to 95 percent.
The effect of particle size on performance of venturi
scrubbers 'is evident in data from two recent scrubber installa-
tions on bark/oil-fired boilers, one with a salt emissions prob-
lem. Because of the small particle size of the salt, the scrub-
ber on the boiler burning salty fuels requires a 15 to 20 in.
H-0 pressure drop to obtain outlet emissions at 0.07 to 0.18
gr/dscf depending on the salt content of the fuel. In contrast,
the other operates at 8 to 10 in. H_0 yielding outlet emissions
of 0.02 gr/dscf.
In crushed stone applications of wet scrubbers, since the
particles emitted from crushers are larger than those from con-
veyors, again a lower pressure drop in the crusher scrubbers will
yield higher collection efficiency. Design requirements from
5-7
-------
Research Cottrell's computer model estimates for crusher appli-
cations are 5 to 16 in. H20 pressure drop to achieve for 99.0 to
99.5 percent efficiency at an L/G of 5 gal/1000 acf. For con-
veyor applications these requirements increase to 8 to 21 in. H?0
pressure drop to achieve 96 to 98 percent efficiency at an L/G of
5 gal/ 1000 acf.
Fabric FiIters
Design A/C ratios for fabric filters presently installed on
bark/fcssil-fuel fired boilers range from 4 to 5:1. The presence
cf fines in the bark mix prevents operation at higher A/C ratio
because cf excessive pressure drops. As an example, the Simpson
Timber Co. baghouse collects ash from salt-laden bark, with a
particle size distribution skewed largely toward the submicron
range. The average operating pressure drop at 10 in. HjO is 3
in. H_0 over the design value. The A/C ratio is probably slight-
ly high at 4:8 to 1. In two other baghouses on bark/'fossi 1-fuel
boilers with no salt emission problem the A/C ratios are 4:1. In
all three installations the use of pulse jet clearing allows for
slightly sm.aller collectors by the increase in A/C ratio. Use cf
a mechanical collector preceding the baghouse is mandatory to
collect hot cinders that could cause fire.
In the crushed stcne industry the baghouse is likely to
remain as the preferred control device because the dry, inert
dust often can be used as a stone product or recycled within the
plant. Particles from the various crushed stone processes are
coarse enough that A/C ratios as high as 7.5:1 are used in con-
5-8
-------
junction with pulse jet cleaning. Shaker cleaning with A/C
ratios of 2 to 3:1 is also popular. Pressure drops from respon-
dents in this study range from 2 to 8 in. H_0; the finer parti-
cles from tertiary crushing and screening cause the higher
pressure drops.
In both industries studied, and in most applications,
fabric filter operation is relatively insensitive to process
variables such as chemical composition, particle size, and re-
sistivity, although chemical composition of particles can cri-
tically affect fabric selection.
5.2 OPERATION AND MAINTENANCE
The more stringent emission standards of recent years
require that a company follow a program of regular maintenance of
control devices to remain in compliance. The high cost of par-
ticulate control equipment also justifies a high priority on
maintenance.
5.2.1 ESP's
Maintenance of ESP's in the kraft pulp mill industry is
apparently more difficult than in utility or metallurgical
applications. On recovery furnaces the most common operating
problems are corrosion and failure of rappers. Other problems
result from drag bottom conveyors, plugging of the inlet dis-
tribution plate, buildup on ladder vanes, and "snowing" or inter-
mittent puffing of recovery furnace stacks. Effluent from low-
odor boilers cause more problems than that from conventional
boilers because of the light, fluffy dust and the high SO-
5-9
-------
content, often causing corrosion. In this application, the
performance of an ESP can deteriorate with time.
Because ESP's are rarely used on bark/fossil-fuel boilers,
little information is available on operation and maintenance.
5.2.2 Scrubbers
With regard to the venturi scrubber, general operational
problems on kraft pulp mill and crushed stone processes parallel
those in other utility and industrial applications. They chiefly
involve ccrrosion, plugging, and abrasion.
Several operators of sludge lime kilns report low mainten-
ance ar.d .-i.-.imal cperatcr attention with venturi scrubbers. How-
ever, in these installations, fresh water should be used instead
of contaminated condensate to minimize odorous emissions from
stripping.
On bark boilers, the performance of a venturi scrubbers is
greatly affected by the quality of the water used. The require-
ment for "bleed-off" of dissolved solids may beccr-2 difficult
because of increasing limits on water discharge.
No data are available on operation and maintenance of
venturi scrubbers on crushers and conveyors in the crushed stone
industry.
5.2.3 Fabric Filters
According to respondents in this study, fabric filters
provide reliable service on crushed stone processes, although
maintenance data are sketchy. Major problems are related to the
cleaning mechanism and dust removal system. Failures of valves
and diaphrams, air compressor bearings and drive belts, and
5-10
-------
solid-state circuits associated with the pulse jet cleaning
mechanism have caused difficulties. Bridging of hoppers and
breakage of screw conveyor shafts by overloading are the two most
common problems in dust removal and conveying systems.
Data from Simpson Timber Co. indicate that overall operation
has been satisfactory; problems center on the collection hoppers,
where the light dust causes plugging. The dust contains salt and
tends to bridge the hoppers. Vibrators have been used success-
fully to relieve the plugging. No reliable sensing system to
detect plugging has been devised.
Data on the effects of various control device malfunctions
on performance of equipment are not available. It can be stated
generally that without regular maintenance the performance of
these control devices will degrade rapidly, especially in the
fine particle size range. This is an area where further study
could be directed in anticipation of particulate emission stan-
dards based on particle size as well as overall mass efficiency.
Additional studies could be directed to deterioration in perfor-
mance of control devices during extended operation, even with
optimum maintenance.
5.3 FRACTIONAL EFFICIENCY RELATIONSHIPS
5.3.1 ESP'S
As expected, computer modeling of the fractional efficiency
of ESP's on kraft recovery furnaces and bark/fossil-fuel boilers
showed a minimum in efficiency in the particle size range of 0.2
to 0.6 ym. This is due to the transition from field charging,
5-11
-------
which predominates for particles greater than 0.5 urn, to collec-
tion by diffusion, which predominates with particles below 0.5
urn. Absolute fractional efficiencies at the same overall mass
efficiency depend very strongly on the inlet size distribution.
The predictions for bark boilers also account for the
effects of re-entrainment, assuming that the fraction of material
remains constant at different particle sizes and for each mechan-
ical section. Minimum collection is still in the 0.2 to 0.4 urn
range.
Fractional efficiency curves based on test data for kraft
pulp mill recovery boilers are available ' for comparison with
predicted results. These test data confirm minimum collection
efficiency in the 0.2 to 0.4 -_m size range and show another
minimum in the 8 to 10 pm range, which could be indicative of
re-entrainment. Another test on an ESP with severe re-entrain-
m.er.t, shows that collection efficiency continues to decline,
leveling off at 70 to 100 '_m. Re-entrainment in the larger
size fractions supports the idea that the dust (most cf which is
re-entrained due to rapping) consists of both large particles and
agglomerates cf smaller particles.
Performance of ESP's on bark/fossil-fuel boilers with salt
emissions was not modeled. The fractional efficiency, however,
probably would be reduced because of the fine salt particles,
with a resultant increase in opacity. No fractional efficiency
test data were available for ESP's on bark/fossil-fuel boilers.
5-12
-------
5.3.2 Venturi Scrubbers
The Research Cottrell computer model for predicting pene-
tration as a function of particle size in venturi scrubbers on
sludge lime kilns, crushers, and conveyors showed the character-
istic deterioration in performance at particle sizes below about
4
2 micrometers. Cheremisinoff's particle data on sludge lime
kilns do not mention the presence of soda fume, which would add
fine particles to the mix and could lead to pressure drops higher
than those presented in the model for efficiencies above 99
percent.
No fractional efficiency test data for venturi scrubbers on
the subject processes could be located for comparison with the
predicted performance models.
5.3.3 Fabric Filters
A prediction model was not available for use in evaluating
fractional efficiency performance of fabric filters. One set of
fractional efficiency test data concerns EPA's mobile fabric
filters applied to a kraft lime kiln. This data supports the
observation that fabric filters maintain collection efficiencies
higher than ESP's or scrubbers in the submicron particle size
range. No fractional efficiency test data are available for
fabric filters applied to bark boilers or crushed stone pro-
cesses.
Fabric filters operating on two bark boilers with a high
percentage of submicron particles have yielded very low opacity
readings, a further indication that the fabric filters are very
efficient collectors of fine particles. Data from the utility
5-13
-------
industry also show that fabric filters can maintain an efficiency
of 99 percent or greater in the submicrometer particle size
range.
5.4 COSTS
Estimates of capital and annual operating costs for ESP's
and wet scrubbers are based on data developed by Research Cot-
trell; estimates for fabric filters are derived from literature
sources. Thus the cost estimates cannot be compared on the same
basis. The intent was to estimate costs for the control device
used credc.~ir.ar.tly en each cf the subject processes. Some gen-
eral conclusions follow.
In the past the almost exclusive use of ESP's on kraft
recovery boilers was dictated primarily by the cost savings in
the recovery of soda ash. This was balanced against the capi-
tal and operating costs over the life of the ESP, at efficiencies
3 8
cf £5 to 95 percent. ' In recent years, however this savings
has been reduced because requirements for higher precipitatcr
efficiencies have led to higher operating costs not cffset by
the increase in rarket value cf soda ash. Although retrofitting
cf venturi scrubbers downstream of an existing precipitatcr may
provide a more economical option, such installations are infre-
quent and thus were not cost-evaluated.
The predominant use of venturi scrubbers on kraft lime kilns
and of low-energy scrubbers on smelt dissolving tanks is dictated
by the relatively higher capital cost of installing ESP's to
handle small gas volumes. A recent comparison of ESP's and
venturi scrubbers on a kraft lime kiln noted that the capital
5-14
-------
investment for an ESP is 3 times greater and that the heat loss
associated with the ESP is too great to overlook. The venturi
scrubber was selected for the lime kiln.
For application to bark/fossil-fuel boilers, fabric filters
are the most expensive control option, followed by ESP's and wet
scrubbers. This observation is based on a recent report by
Weyerhauser Corp., which estimates the costs of installing all
three devices on a bark/fossil-fuel boiler with salt emissions.
Comparison of fabric filter and venturi scrubber applica-
tions on crushed stone processes shows that capital installed
costs for venturi scrubbers are slightly higher and that opera-
tion and maintenance costs would be approximately 1-1/2 times
those for fabric filters.
West dust suppresion can be used on primary crushers,
screens, transfer points, and crusher feeds, where particle sizes
are larger and moisture content is higher, in combination with
fabric filters at points where fine particle emissions occur,
primarily secondary and tertiary crushers and screens. This
results in greater emission reduction than complete wet dust
suppression, and is more economical than a complete fabric
filter system.
5-15
-------
REFERENCES - SECTION 5.0
1. Figger.bach, J.D. et al. Fine Particle Emissions Information
System Series Report, Test Series No. 13, Environmental
Science Inc., 1973.
2. Gooch, J.P. et al. Farticulate Collection Efficiency Mea-
surements on an Electrostatic Precipitator Installed on a
Paper Mill Recovery Boiler. Southern Research Institute,
EPA 600/2-76-141, May 1976.
3. Nichols, Grady B. Particulate Emission Control From Pulp
Mill recovery Boilers With Electrostatic Frecipitators , in
IZEE Transactions on Industry Applications, Vol. 1A-13, No.
1, January/February 1977.
4. Cheremi sir.off, P.N., and R. A. Young, Air Pollution Control
and Des_i_g_n Handbook. 1977, page 841. ' '
5. Fine Particle Emissions Information Systems Series Report,
Test Series Nos. 105, 106, and 107, selected runs, Monsanto
Research Corp., EPA Contract No. 68-02-1816, 1975.
6. Lee, David R. An Econo-ic Comparison of Kiln Particulate
Control Alternatives: Electrostatic Precipitators vs.
Venturi Scrubbers, Long view Fibre Company, presented at
Proceedings of the 1976 NCASI West Coast Regional Meeting.
NCASI Special Repcrt No. 77-04, June 1977.
7. See, R.C. North Bend Hog Fuel Boiler Emission Collection
Options. Weyerha-ser Co., April 1978.
8. Rust, J.P. Application of Electrostatic Precipitators for
the Control of Fumes From Low Odor Pulp Mill Recovery
Boilers. J. Air Pollution Control Associawtion 25 ( 2) : 158-162 ,
February 1975.
5-16
-------
APPENDIX A-l
INSTALLATION LISTS FOR PARTICULATE CONTROL DEVICES ON
KRAFT PULP MILL APPLICATIONS
A-l
-------
Table A.1-1.
SELECTED
F-ESEARCH-COTTRZLL, INC.
PULP KILL RECOVERY PRECIPITATORS
A. HORIZONTAL FLOW WET BOTTOM TYPE
No., Type & Size No. of Total Gas
Recovery Units Pptrs. Flow (CFM)
Lbs. BLS.a/Day & Shell Gas Ter.o.
Installation Pulp Equivalent Material (WF)
Y-5S13S1-C1
64C T/Day
lor.solidated Pontiac,
Tahsis Ccr,pa_-w, Ltd. 1 360,000
Geld River, British 3,150,000 Hollow Tile 325
Col-r-cia, Canada 1350 T/Day
International Pacer
Oo. " 1
Moss Point 1,900,000 Steel
Alabama Kraft Co. 12
Fncenix City 2,750,000 Filled
Alacar.a " 915 T/Day Tile
Soott-y.aritine Paper 191,000
Co. 1,725,000 1 30C
Abercr-r±ie 575 T/Day rilled
Nova Scotia Tile
Canada
MarMillan & Blcecel 1 255, C
Lir.it e= 2,250,OOO Filled 325
Powell River, Br. 750 T/Day Tile
C~l_~::ia, Canada
1,650,COO Filled 3::
~uecec, Canada 550 T/Day Tile
Owens-Illinois Corp. 2 360,000
Orange (2) x 1,650,000 ea. Filled 300
Texas (2) x 550 T/Day ea. Tile
In t e r = ; n 11 n en t a 1
Pulp Co., Ltd 1 315,000
?r-noe George, B.C. 2,550,000 Steel 3CC
Canda 850 T/Dav
aBLS = Black Liquor Solids
(continued)
A-2
-------
Table A. 1-1 (continued).
Installation
Tennessee River
Pulp & Paper Co.
Counce, Tenn.
Consolidated Papers
Inc.
Wisconsin Rapids
Wisconsin
Eastex, Inc.
Evadale, Texas
Chesapeake Corp.
of Virginia
West Point, VA
Skookumchuk Pulp
Cranbrook, B.C.
Potlach Forests
Lewiston, Idaho
Continental Can
Co.
Hopewell, Va.
Kimberly Clark
Mexico
U.S. Plywood
Champion Papers
Courtland, Ala.
W. Va. Pulp &
Paper Co.
Wickliffe, Kentucky
No. , Type & Size
Recovery Units
Lbs. BLS.a/Day &
Pulp Equivalent
1,250,000
415 T/Day
1,200,000
400 T/Day
No. of
Pptrs.
Shell
Material
1
Filled
Tile
1
Filled
Tile
Total Gas
Flow (CFM)
Gas Temp.
173,000
325
175,000
300
1,600,000
530 T/Day
2,400,000 bis/
day
2,100,000
700 T/Day
900,000
300 T/Day
1,050,000
350 T/Day
360,000
120 T/Day
Hollow
Tile
2 Pilaster
Filled Tile
1
Tile
Shell
1
Hollow
Tile
1
Steel
Cstainless)
1
Steel
Pilaster
Tile Shell
Pilaster
Filled Tile
Pilaster
190,000
245,000
325
248,000
300
104,500
325
175,000
250
46,600
325
250, OOO/
pptr.
270,000
BLS = Black Liquor Solids
(continued)
A-3
-------
Table A.1-1 (continued).
Installation
Boise Cascade
De Ridder, La.
Southland Paper
Mills, Inc.
Lufkin, Texas
St. Mary's Kraft
Division
C-ilr.ar. Paper Co.
St. Marys, Ga.
No., Type & Size
Recovery Units
Lbs. BLS. /Day &
Pule Equivalent
NO. of
Pptrs
Shell
Material
Total Gas
Flow (CFM)
Gas Temp.
'
1 303,000
Filled Tile
Pilaster
Pilled
Tile
Filled Tile
Pilaster
140,000
170,000
Total
rontinental Can
Co.
Hopewe 11 / \'n -
Paper Co.
Erie, Penna.
•^...^.-gv-wji*.^ r"1 *•* a ^ "D a ^ o T*
* . ^ _ — ^ . * a _.*.w**3.^ * 3 ^ C *
Co.
Texarkar.a, Texas
Steel Heat
Jacket
Steel Shell
Steel Shell
Opzel
21 C , 0 0 0
23:
3: =, o o o
acfn
250,000
325
ELS = Elack Liauor Solids
(continued!
A-4
-------
Table A.1-1 (continued)
B. DRAG BOTTOM TYPE
Installation
American Can Co.
Halsey, Oregon
International
Paper Co., Inc.
Glens Falls
New York
Weverhaeuser Co.
No., Type
Kraft Recovery
Kraft Recovery
1
Springfield, Oregan Kraft Mill
Unit #4
Hoerner-Waldorf
Corp.
Missoula, Montana
Boiler *3
Western Kraft Corp.
Continental Can
Hodge, Louisiana
Salt Cake
Salt Cake
Collection
(Controlled
Odor)
No. of
Pptrs.
Shell
Material
1
Steel
1
Steel
Opzel Filled
Tile -
Pilaster/
Plenums
1
Steel
1
Opzel
Steel
Total Gas
Flow (CFM)
Gas Temp.
164,240
180,000
249,000
207,000
400
230,000
365
390,000
340
A-5
-------
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A-6
-------
Table A. 1-3. INSTALLATION LIST FOR BARK/FOSSIL FUEL BOILERS'"
Sic
26ii
26ii
2611
2611
261
261
261
26i
26i
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^ELHAMIAL CuLLECTuR
MELHAMCAL tl/LLELlUR
MECHANICAL Cl'LLELtliR'1 '
•-ISCELLANEIlUS
MECHANICAL CULLELTUR
MECHANICAL CULLELlllR
MfcCHAMCAL CULLELTi.R
MECHANICAL CULLELUlK
MtLMANICAL CULLECTux
MECHANICAL COLLECTOR
MECHANICAL CULLECTUR
ME I SCRUHHE.R
MECHANICAL CULLECTUR
MECHANICAL CULLECTUR
MECHANICAL CULLELTUR
MECHANICAL COLLECTOR
MECHANICAL COLLECTOR
MECHANICAL CDLLECtUS
MtCHAMLAL LUlLlLlu1*
MECHANICAL COLLECTOR
MECHANICAL COLLECTOR
MECHANICAL COLLECTOR
MECHANICAL COLLECTOR
MECHANICAL CULLELTUR
»E 1 SCRUrihE R
*E T SLRUE>Ht-R
HE 1 SCRUMMfR
MISCELLANEOUS
ELtLIRU STATIC PREC1HI IA MlR
MECHANICAL COLLECTOR
MlSCELL»NEllU!>
Ml SLILL ANtoUb
Ml iCELLANtllOb
MECHANICAL COLLECTOR
MECHANICAL LULLEC'OR
MECHANICAL CULIECTllR
MECHANICAL COLLECTiPK
M SCELLANtoOa
»fL«AN|LAl luLLECTOR
Ml iCELLAKF.OOS
MfcMAMCAL CIICLELTOR
MMLHANllAl CIICLLLTUR
fECHAMLAL fuLLfLlUR
Eff.
bv.o
9U.O
63.0
50.0
80.0
90.11
9t'.0
96. U
9U.U
VS.O
uo.o
so. u
8b.O
Bb.O
8b.O
.0
.0
.0
.0
90.0
92.0
90.0
93.0
•jb.o
•»u.o
•40.0
90. 0
65.0
/S.o
041. (1
7b."
75.0
85.0
85.0
05.0
75.0
75. u
/5.U
B«.0
99.0
99.0
7U.O
IV.V
91.0
90.0
90. U
9e!.iJ
92. U
9b.ii
95.1'
75."
9ii.O
(45. U
0-l.n
A-7
-------
Table A.1-3 (continued!
SlC
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<•»H
ti(\
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(KC 1
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ioe \
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* U^ li-ftLU 3"*5««
»L.S;i«ttP»^t'»CufiK ^••^)'>
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1 w t^ U w ** u ^ C^*>3JL]C*'tL •* * P t * i win A*t N ^ * w 4 «
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u»fxS-lLl. >t«£»P f'uD Cl» !_-»-«. I •>.-•/
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'» LM4XICIL CuLLt L l"«
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lit.
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A-8
-------
Table A.1-3 (continued)
Sic
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?bii 1
t\
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£b£l
lbt\
it>d\
eotn
ibtl
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ttai
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dbiM
£bt!i
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£•631
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£b3i
ibil
ttil
£b3l
ludUUYud
10i»uu*u
1 iKJOU^Uf
1 (ttUUVUl!
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buntltKS MPtK tO C»LMOUH 17309
M«UUi» tU"»xu!> lav ulSL RlvEK UK }« KlOF) tllv N L HAHtEilU'V £f«U6
111 K » T U M t k ftxt, »NUlU» b t * L « 3£U3U
Ml h/HuMtk ftKN«M)II4* btAL" StfUlU
INTNL. HAPtK CU, b.7Ub9,K iNt BLU^^" 3il)ulb
TKlLMANT HUl.P»P*HtK CU K•U^*U^» 5KI3U
FIBKteOtKU (.UxH MlLBUM »Vt AM1ULH
fL»MBt»U PAt-E" CUifuO FlKSl A»£ N , 5Mib<;
FLAMBtAU PAPEW CU,?UO ^lKSl »Vt H, b«bbt
A»tH]CAN CAN CU t-t Nr< 1 nlj 1 Ul. ib4U4
l>•t»•lCA^ CAf, CO Pt NNJootON 3»< "36(1 lULtUU
ttUKbIA PACIFIC COMPPU BU» SBU lOLtOO
IM'L PAPt« CU LA. MILL BASlKUP LA 7l£«ri;
CUNIINENIAL CAN PUBIIUOo AUl,U!>TA 3UVU3
CuNllNEMAL CAN CU fun in t-Ul nUPEntLLBbU
• t i I t Hf, ' R>( AM CUWH I-S AT NUHth ALBANY
CHtSAPtAKt LUKPUKAIIUN UF »A »[bl PUlNl
MLNAbnA CUKfUKAIIUn JUHL/Ar. PUiNI, NOMIh
MtNAbHA CUMPUHAIIUN JUHI;AI. H01.il, NUMIH
IKLANli LUNlAI'.en BI <:*>-! MEn JiJnNV 37)5«
HuEKNtK hALUUKf CUKP 111 LAnESMOKE KUAl)
INLAND CUN!AH«EH Bi £4s r,E« JJHNV 3713M
MAMMthHILL ^APtX CU-15«3 t LAHE KJ-tlHt
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ll«l t KNA 1 1UNAL PAPtM, uttO
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INTEKNA1IUNAL PAffcH, HEtO
bUNULU t-KOUUCIS CU HAHlsVlLLt S C £>«3bU
ABIIIBl CUKI- HuAHlKb HlVt>
UMUN LAMP tLub HKHI ulv BU< I fa FNANKLN
B-cap.
Mlb/hr
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4 4u
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376.
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Icfu.
111.
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b£7.
£9b.
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l£u.
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b7.
luu.
Type of control
M.CHAMLAL LULLLLIUK
KtLMAulLAL LULLkLlt'K
MtCMANKAL CULLH.TUK
Ktl bLHUodtH
fct 1 SCXlJDUt*
^fCMANILAL CuLLtCtUK
KtLMANlLAL CULLtLlUK
HtCMAMCAL LULLtLll/N
Wl 3Ct LL AI.EuUb
KtCHANlCAL CuLLELIUH
KtCMAHlLAL LULLtLlUK
htChAKjCAL CULLtLlUt*
MlbLtLLANtUUb
fl SCtLLANtuui
htCMAMLAL CULLECIUri
HtUnAi.ILAL CULLtCluK
Kl bLELL ANt UUb
MECnAMCAL LULLtLTlJK
MtCnAlvlCAL CULLtLlUK
KtCHANlCAL CULLtLTUK
MECHANICAL CULLfClUW
MECHANICAL CULLELIUH
MliCELLA,,tUU5
M SCELLANEUUS
Mtl SCMUBBtM
MtLHANlCAL CULLtClUR
MECHANICAL CULLiCIUH
MECHANICAL CULLELlUK
MECHANICAL CULLtCluK
MECHANICAL CULLlLluK
MECHANICAL CULLtLlUK
KE 1 SLKUBBtK
RE I iCKUBBtK
MECHANICAL CULLEC1UH
MECHANICAL CULLELlUK
ELtClhu STAlJC PKtCIHMAIUK
KECnAMCAL CULLELTuM
MtLHANlLAL CULL ELI UK
MECHANICAL CULLEClUK
KECHAMCAL CULLtClUK
KELHAN1LAL CULLELlUK
MECHANICAL LULLELlUK
Ml aCtLLANt uUi
KtL'HANlLAL CULLEClUK
MlSCtLLANEUUb
MECHANICAL CULLELIUH
MECHANICAL CULLELlUK
MLhAMCAL CULLELlUK
MLHAKlCAL CULLELlUK
MtC«Ai.ILAL CULLELlUK
MECHANICAL CliLLECTtlM
KtLHAnlCAL CuLLtLltiK
MlbLELLANtDub
1-El.HANjLAL CuLLLLlux
Eff .
03. u
**u . u
Vx.u
4U.U
vo.u
xS.u
1B.U
So.u
7b.U
n.v
07. u
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VU . I*
•Jb.u
«v. u
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ab.u
ob.u
V3.U
70. u
ob.u
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eb.u
su.u
vu.u
9£.u
eb.u
vb.u
•«i.u
VB.U
vu.u
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w.u
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MII.U
•Ji.u
A-9
-------
APPENDIX A-2
CAPITAL AND ANNUAL COSTS OF PRECIPITATORS FOR
PULP MILL APPLICATIONS
A-10
-------
5000
4500
4000
3500
*•>
o
* 3000
*^
i—
z
LO
£ 2500
S 2000
a.
1500
1000
500
99.9% EFF,
_L
JL
100 200 300 400
GAS FLOW RATE, ACFM x 10
500
3
600
700
Figure A.2-1. Capital investment for precipitators
on conventional (high—odor) recovery furnaces.
A-ll
-------
1000
900
800
700
Is 600
X
«*
- - -
t ^
o
_i
I 400
inn
w -
100
99.95 EFF,
100
200
300
400
500
600
700
FLOW RATE, ACFM x 10
Figure A.2-2. Annual costs for precipitators on
conventional (high-odor) recovery furnaces.
A-12
-------
<*•>
o
5000
4500
4000
3500
3000
2500
£ 2000
o.
1500
1000
500
99.8% EFF.
97.9% EFF.
96% EFF.
J
100 200 300 400
GAS FLOW RATE, ACFM x TO
500
3
600
700
Figure A.2-3. Capital investment for precipitators
on low-odor recovery furnaces.
A-13
-------
o
LJ
_J
«s
1000
900
SDC
60C
43;
3 DC
100
99.8i EFF.
97.9%
100
200
300
400
dAS FLOW R*TE, ACFM x 10
500
3
600
700
Figure A.2-4. Annual costs for precipitators on
low-odor recovery furnaces.
A-14
-------
5000
4500
4000
3500
3000
2500
S 2000
1500
1000
500
99.9% EFF.
99% EFF.
98% EFF.
I
100 200 300 400
GAS FLOW RATE, ACFM x 10
500
3
600
700
Figure A.2-5. Capital investment for precipitators on
bark combination bark fossil fuel-fired boilers.
A-15
-------
3
1000
900
soo
700
600
100
99.9i EFF.
99i EFF.
98i EFF.
100
200
300
400
500
600
700
GAS FLOW RATE, ACFM x 10
Figure A.2-6. Annual costs for precipitators on bark
corhination bark fossil fuel-fired boilers.
A-16
-------
APPENDIX A-3
CAPITAL AND ANNUAL COSTS OF VENTURI SCRUBBERS ON
KRAFT PULP MILL AND CRUSHED STONE INDUSTRY PROCESSES
A-17
-------
Q_
•a:
600
500
400
300
100
30
T » 500eF, L/G « 10
£ * 4.2 °g « 3.Z3
60 90
GAS RATE, ACFM x 10
120
Figure A.3-1. Effects of collection efficiency ana gas
rate on the capital ir.vestrer.t of venturi scrubber systems
for lime kilns.
A-18
-------
300
o
x
1/5
O
<_>
C5
2
UJ
Q.
O
:D
z
«£
200
100
- 90% COLLECTION EFFICIENCY /
--- 95% COLLECTION EFFICIENCY '
OTHERS INCLUDE THE COST OF /
WATER LABOR AND MAINTENANCE,/ T = 50o°F L/G = 10
ETC. /
x = 4.2, °g = 3.23
\x
*v
<$*'
<&
<$>'
$&
£'
&'
^~— OTHERS
30
60
90
120
GAS FLOW RATE, ACFM X 10
Figure A.3-2. Effects of gas rate, electricity usage,
fixed charges, and others on the annual costs of venturi
scrubber systems for lime kilns.
A-19
-------
800
700
^ 600
o
X
z;
t—
t/i
UJ
=»
500
400
300
200
100
30
Oi salt, 2% ash
T = 390°F. L/G « 7
x = 15, cg « 4
60
90
120
150
180
GAS FLOW RATE (acfm x 10 )
Figure A.3-3. Effects of collection efficiency and cas rate
on the capital investment of venturi scrubber
systerr.s for bark/fossil fuel boilers.
A-20
-------
300
CO
o
IS)
o
o
200
TOO
92% COLLECTION EFFICIENCY
91% COLLECTION EFFICIENCY
OTHERS INCLUDES THE COST OF
WATER, LABOR AND MAINTENANCE,
ETC.
0% SALT, 2% ASH
T = 390°F, L/G = 7
x = 15, °g = 4
ED CHARGES
•ELECTRICITY
I
I
30
60 90 120 150
GAS FLOW RATE (acfm x 10 3)
180
Figure A.3-4.
Effects of gas rate, electricity usage,
fixed charges and others on the annual
costs of venturi scrubber systems for
bark/fossil fuel fired boilers.
A-21
-------
o
X
800,
700
600
500
400
300
200
TOO
1.5e salt, high ash
T = 400'F, L/G « 8.5
x » 0.38, °g « 7.1
30
60
90
120
150
180
GAS FLOW RATE (acfm x 10 J)
Figure A.3-5.
Effects of collection efficiency ar.d gas
rate on the capital investment of venturi
scrubber systems for bark/fossil fuel
fired boilers.
A-22
-------
300
o
x
I/O
o
o
200
TOO
56% COLLECTION EFFICIENCY
52% COLLECTION EFFICIENCY
OTHERS INCLUDES THE COSTS OF -.
WATER, LABOR AND MAINTENANCE
ETC.
1.5% SALT, HIGH ASH
T = 400°F, L/G = 8.5
x = 0.38 °g = 7.1 &/ s
<§S /
^
^Y ^
FIXED CHARGES
ELECTRICITY
30
60
90
120
150
180
GAS FLOW RATE (acfm x 10
Figure A.3-6.
Effects of gas rate, electricity usage,
fixed charges and others on the annual
costs of venturi scrubber systems for
bark/fossil fuel fired boilers.
A-23
-------
300
o
X
200
«s
I—
o.
IOC
T - 75'F, L/G « 5 J - 200, °g « 8.7
10
20
30
40
GAS FLOW RATE, ACFM x 10
Figure A.3-7. Effects cf collection efficiency and gas
rate on the capital investment of venturi scrubber systems
for stone crushers.
A-24
-------
CO
o
a.
o
99% COLLECTION EFFICIENCY
99.5% COLLECTION EFFICIENCY
OTHERS INCLUDE THE COSTS OF WATER,
LABOR AND MAINTENANCE,' ETC.
TOO
° 80
X
s 6°
o
40
20
I I
75°F, L/G = 5
200, °g = 8.7
I
FIXED CHARGES'
i— — ***** ^*-~
10
20
30
40
GAS FLOW RATE, ACFM x 10
Figure A.3-8. Effects of gas rate, electricity usage,
fixed charge, and others on the annual costs of venturi scrubber
systems for stone crushers.
A-25
-------
300
200
TOO
= 75°F, L/G « 5 x « 10, cg - 4
10
20
30
GAS FLOW RATE, ACFM x 10
40
Figure A.3-9. Effects of collection efficiency and gas rate
on the capital investment of venturi scrubber systems
for stone crushing conveyors.
-------
ro
O
O
O
QC
UJ
O.
O
96% COLLECTION EFFICIENCY
98% COLLECTION EFFICIENCY
OTHERS INCLUDE THE COSTS OF WATER,
LABOR AND MAINTENANCE, ETC.
100
80
60
40
20
I I I I
75°F, L/G « 5 x = 10, °g « 4
10
20
30
40
GAS FLOW RATE, ACFM x 10
Figure A.3-10. Effects of gas rate, electricity usaqe,
fixed charges^, and others on the annual cost of
venturi scrubber systems for stone crushing conveyors.
A-27
-------
APPENDIX B-l
ELECTROSTATIC PRECIPITATOR SUBSYSTEM AND
COMPONENT FUNCTION AND OPERATION
B.I.I TRANSFORMER-RECTIFIERS
The transformer-rectifier unit consists of a high-voltage
transformer, high-voltage silicon rectifiers, and high-frequency
choke coils. The unit converts the low-voltage alternating
current to high-voltage unidirectional current suitable for
energizing the precipitator.
The transformer, rectifiers, and choke coils are submerged
in a tank filled with a dielectric fluid. The tank is equipped
with high-voltage bushings, liquid level gauge, drain valve,
ground lug, filling plug, lifting lugs, and surge arresters,
which discharge any harmful transients appearing across the dc
metering circuit to the ground.
The electrical equipment described below comprises the
components necessary to produce and control the high-voltage
unidirectional power required to energize the electrostatic
precipitator. The transformer-rectifier and control unit provide
a complete system for energizing with either half-wave or full-
wave voltages. Not all precipitator installations incorporate
all of these subcircuits, but most will include many of these
features; some of the automatic features described below may be
done manually on some installations.
B-l
-------
A subsystem that automatically maintains and limits optimum
current and voltage to the high-voltage transformer, which
is connected to the discharge wires.
Silicon controlled rectifiers (SCR's) that provide a wide
range of precipitator current and voltage control.
A current-limiting reactor that limits current surges
during precipitator sparking.
Automatic restart to initiate system operation after a line
voltage failure or temporary ground condition in the pre-
cipitator.
Overload protection for the high-voltage rectifiers.
Panels containing component modules; the SCR power circuit,
dc overload circuits, relays, control transformers, resistors,
m.ain contactor, and current transformer and other components are
mounted in the control cabinet and are completely accessible for
servicing. Positive ventilation for the control unit is provided
by an intake fan located near floor level. Ventilating air is
exhausted through an opening (grill-protected) in the upper rear
of the control unit.
The transformer enclosure is a square metal housing bolted
to the top of the tra-.s f crrer tank. The enclosure protects the
transformer bushings and electrical connections from weather and
also ensures, via a key interlock system, that none of the elec-
trical connections or bushings can be handled until the associ-
ated control cabinet has been de-energized and grounded.
The transformer pipe and guard are used to feed the high-
voltage output of the transformer-rectifier to the support bush-
ings, which in turn are connected to the upper high-tension
support frame, from which the discharge wires are suspended.
B-2
-------
Figures B.l-1 and B.l-2 illustrate vibrator rapper and insulator
assemblies and their relationship to the rest of the precipitator
system.
B.I.2 AUTOMATIC POWER CONTROLS
During normal operation, optimization of applied power to
the precipitator is accomplished by automatic power controls,
which vary the input voltage in response to a signal generated by
the sparkover rate. Provisions are also included to make the
circuit current-sensitive to overload and to allow control in the
event that spark level cannot be reached. Although the circuits
may vary among installations, many of the features described
below are common. An SCR mainline control diagram is presented
in Figure B.l-3 to illustrate operation of the automatic power
control system.
When the circuit breaker and control circuit on/off switch
are closed, power flows through the current-limiting reactor,
current transformer, and current signal transformer to the pri-
mary of the high-voltage transformer. The SCR's act as a vari-
able impedance and control the flow of power in the circuit. An
SCR is a three-junction semiconductor device that is normally an
open circuit until an appropriate gate signal is applied to the
gate terminal, at which time it rapidly switches to the con-
ducting state. Its operation is equivalent to that of a thyro-
tron. The amount of current that flows is controlled by the
forward blocking ability of the SCR's. This blocking ability is
controlled by the firing pulse to the gate of the SCR. The
B-3
-------
DISCHARGE
ELECTRODE
VIBRATOR
DISCHRAGE
ELECTRODE
VIBRATOR
DISCHARGE ELECTRODE VIBRATOR
AND INSULATOR ASSEMBLY
COLLECTING
ELECTRODE
RAPPER
COLLECTING ELECTRODE RAPPER
AND INSULATOR ASSEMBLY
Figure B.l-1. Insulator, vibrator-rapper assembly, and
precipitator high-voltage frajne.
B-4
-------
VIBRATOR
OR RAPPER
BRACKET
UPPER RAPPER ROD
POWER CABLE
STUFFING BOX
SEAL PLATE
ASBESTOS PAD
INSTALLATION
ACCESS DOOR
LOCATED TO SUIT
HIGH TENSION
DUCT CONNECTION
LOCATED TO SUIT
INSULATOR COMPARTMENT
VENTILATING OR PRESSURIZING
AIR CONNECTION-LOCATED TO SUIT
— INSULATOR SHAFT
ASBESTOS PAD
INSTALLATION
LOWER RAPPER ROD
SUPPORT BUSHING
PRECIPITATOR ROOF
Figure B.l-2. Precipitator insulator and vibrator-rapper
assembly.
B-5
-------
C
u
4J
C
C
O
O
C
a
u
B-6
-------
current-limiting reactor reshapes the current wave form to
essentially a sine wave and limits peak current due to sparking.
The firing circuit module provides the proper phase-con-
trolled signal to fire the SCR. The timing of the signal is
controlled by 1) the potentiometer built into the module, 2) the
signal received by the automatic controller, and 3) the signal
received by the spark stabilizer. .
The automatic control circuit performs three functions:
spark control, current-limit control, and voltage-limit control.
Spark control is based on storing electrical pulses in a
capacitor for each spark occurring in the precipitator. If the
voltage of the capacitor exceeds the preset reference, an error
signal will phase the mainline SCR's back to a point where the
sparking will stop. Usually this snap-action type of control
will tend to overcorrect, resulting in a longer downtime than is
desirable. At low sparking rates, about 50 sparks per minute,
the overcorrection is more pronounced, resulting in reduced
voltage for a longer period, with subsequent loss of dust and
reduced efficiency.
Proportional control is also based on storing of electrical
pulses for each spark occurring in the precipitator. The phase-
back of the mainline SCR's, however, is proportional to the
number of sparks in the precipitator. The main advantage of
proportional control over spark control is that the precipitator
determines its own optimum spark rate, based on four factors:
temperature of the gas, dust resistivity, dust concentration, and
-------
internal condition of the precipitator. In summary, with pro-
portional spark rate control, the precipitator determines the
optimum operating parameters. With conventional spark control,
the operator selects the operating parameters, which may not be
optimal.
Some precipitators operate at the maximum voltage or current
settings on the power supply with no sparking. In collection of
low-resistivity dusts, where the electric field and the dust
deposit are insufficient to initiate sparking, the r.c-spark
condition ray arise. The fact that the precipitator is not.
sparking does net rean necessarily that the unit is underpowered.
The unit may have sufficient power to provide charging and elec-
tric fields without sparking.
The voltage-limit control feature of the automatic control
module limits the primary voltage of the high-voltage transformer
to its rating. A transformer across the primary supplies a
voltage control, as in the case of the current limit. The vol-
tage control setting is adjusted for tne primary voltage rating
cf the high-vcltage transformer. When the primary voltage ex-
oeeds this value, a signal is generated that retards the firing
pulse of the firing module and brings the primary voltage back to
the control setting.
For current-limit control, a transformer in the primary
circuit of the high-voltage transformer monitors the primary
current. The voltage from this transformer is compared with the
setting of the current control, which is adjusted to the rating
B-8
-------
of the transformer-rectifier unit. If the primary current ex-
ceeds the unit's rating, a signal is generated, as with spark
control, which retards the firing pulse of the firing circuit and
this brings the current back to the current-limit setting.
With all three control functions properly adjusted, the
control unit will energize the precipitator at its optimum or
maximum level at all times. This level will be determined by
conditions within the precipitator and will result in any one of
the three automatic control functions operating at its maximum,
i.e., maximum voltage, maximum primary current, or maximum spark
rate. Once one of the three maximum conditions is reached, the
automatic control will prevent any increase in power to reach a
second maximum. If changes within the precipitator so require,
the automatic control will switch from one maximum limit to
another.
Other features include secondary overload circuits and an
undervoltage trip capability in the event that the voltage on the
primary of the high-voltage transformer falls below a predeter-
mined level and remains below that level for a period of time. A
time-delay relay is also used to provide a delay period in the
annunciator circuit while the network of contacts is changing
position for circuit stabilization due to an undervoltage con-
dition.
B.I.3 VIBRATORS
The purpose of a vibrating system is to create vibrations in
either the collecting plates or the discharge wires to dislodge
B-9
-------
accumulations of particles so that the plates or wires are kept
in optimum operating condition.
The vibrator is an electromagnetic device, the coil of which
is energized by alternating current. Each time the coil is
energized, the vibration set up is transmitted to the high-
tension wire supporting frame and/or collecting plates through a
rod. The number of vibrators depends on the number of high-
tension frames and/or collecting plates in the system.
The control unit contains all devices for operation of the
vibrators, including means of adjusting the intensity of vibra-
tion and the vibration period. Alternating current is supplied
to the discharge wire vibrators through a multiple cam-tyre timer
to provide the sequencing and time cycle for energization of the
vibrators.
For each installation, a certain intensity and tine period
of vibration will produce the best collecting efficiency. In-
sufficient vibrating intensity will result in heavy buildups of
d-ist on the discharge wires which can cause the following adverse
operating conditions.
It reduces the spark-over distance between the electrodes,
thereby limiting the power input to the precipitatcr.
It tends to suppress the formation of negative corona and
the production of unipolar ions required for the precipita-
tion process.
It alters the normal distribution of electrostatic forces in
the treatment zone. Unbalanced electrostatic fields can
cause the discharge wires and the high-tension frame to
oscillate.
B-10
-------
B.I.4 RAPPERS
The rapper equipment is a completely electrically operated
system for continuously removing dust from the collecting plates
within the precipitator. The system is composed of a number of
magnetic-impulse, gravity-impact rappers that are periodically
energized to rap the collecting plates for removal of dust de-
posits. The main components of the system are the rappers and
the electrical controls.
The magnetic-impulse, gravity-impact rapper is a solenoid
electromagnet consisting of a steel plunger surrounded by a
concentric coil, both enclosed in a watertight steel case. The
control unit contains all the components (except the rapper)
needed to distribute and control the power to the rappers for
optimum precipitation. The electrical controls provide a number
of separate adjustments so that all rappers can be assembled into
a number of different groups, each of which can be independently
adjusted from zero to maximum rapping intensity.
B.I.5 UPPER PRECIPITATOR
On positive or negative pressure installations a pressuri-
zing fan is supplied (located on the cold roof) to force air into
the top housing and down through the support bushings. This air
prevents the process gases in the precipitator from entering the
top housing and contaminating the support and high-tension frame
vibrator-rapper insulators. Electric heaters are also used to
prevent condensation buildup on the porcelain bushings, thereby
preventing electrical short circuits which may damage the bushings,
B-ll
-------
In place of a top housing, some installations have insulator
compartments. The insulator compartment is a steel enclosure
that surrounds the high-tension frame support insulators and
rapper rod insulators. Fans are provided to prevent condensation
of noisture on the high-voltage support insulator, and sometimes
electric heaters are installed near each bushing in each insu-
lator compartment.
The purpose of the high-tension anvil beam, which is part cf
the high-tension frame, is to transfer the impact of the high-
ter.sicn vibrator to the discharge wires.
3.1.6 DISCHARGE WIRES
The discharge electrodes are small-diameter wires suspended
from a structural steel wire supporting frame, held taut by
individual cast iron weights at the lower end and stabilized by a
steadying frame at the top of the cast iron weights. Unshrouded
and shrouded discharge wires are illustrated in Figures B.l-4 and
B.l-5, respectively.
~ •« -. ,-.^T»~«r~
-------
— SHROUD CAP
— SHROUD
WIRE
SHROUD
Figure B.l-6. Precipitator
collecting electrodes
Figure B.l-4. Discharge
electrode unshrouded
CAST
IRON
WEIGHT
Figure B.l-5.
Discharge electrode
shrouded
B-13
-------
B.I.8 LOWER PRECIPITATOR
The lower steadying frame limits or restricts the horizontal
movement of the discharge wires.
B-14
-------
APPENDIX B-2. PRECIPITATOR PREOPERATION CHECKLIST
1.) General
Before start-up of the precipitator(s) and auxiliary
equipment, a complete check and visual inspection of
the following items should be performed.
2.) Precipitator
a) Duct spacing .
b) Collecting plates
0 Bowing
e Bellying
0 Supports
0 Spacer bars
0 Corner guides
c) Gas sneakage baffles
d) Anti-swing devices
e) Hoppers
0 Dust level indicators
0 outlet connections
0 Access doors
e Poke holes - anvils
0 Vibrators
f) Insulator housing
Support bushings
Access doors
Ventilation system
Bushing connections
Bushing heaters
Check Initial Date Recheck Remarks
B-15
-------
Check
Initial
Data
Fecheck
Remarks
g) Flues
• Nozzle connections
• Expansion joints
• Louver dampers
• Guillotine dampers
• Perf. distribution
plates
h) Line voltage
• 460/480 vclts-60 Hz
• 575 volts - 60 Hz
0 120 volts
• Line matching transformer
i) Discharge electrode wires
• Upper steadying frame
• Lower steadying frame
• Hanger pipes
• Lifting rods
• C.I. weishts -
15 25 ' 35
j) High-tension guard
• Installation
• Vent ports open
• Ground connections
k) Drag bottom conveyor
1) Wet bottom agitators
m) Keat packet system
• Fecirculating fan
• Electric heater - kW
• Steam heater coils
• Temperature transmitters
• Pneumatic recorders
• Steam, control valve
• Starters - pushbuttons
• Thermostats
fi) Roof enclosure
• Ventilation
* Air conditioning
* Monorail system
• Roof exhausters
• Louvers
• Heaters
B-16
-------
o) Gaskets for high
temperature
3.) Auxiliary Equipment
a) Transformer-rectifier
units
e
o
o
0 O
o o
o o
o o
o o
o o
o o
o o
Surge arrester gap
Transformer liquid level
Ground connections
Precipitator
Transformer
Rectifier
H.T. bus duct
Conduits
FW/HW switch box
Alarm connections
Contact making
thermometer
Ground switch
operation
High-voltage connections
Telephone jacks
Sound power jacks
Resistor board
Space heaters
b) Rectifier control units
0 Controls grounded
e Connections to
equipment
0 Space heaters
e Internal light and
switch
e Alarm connections
0 Space heaters
c) Rapper control unit
Connections
Lights
Space heaters
d) Vibrator control unit
• Connections
• Lights
0 Space heaters
Check
Initial
Data
Recheck
.
Remar
B-17
-------
e)
f)
h)
i)
3>
k)
1)
m)
n)
o)
P)
q)
F.D. Ventilation
controls
• Motor
• Starters
• Pushbutton stations j
• Alarm connections
0 Filters
Electric heater controls
• Hoppers
0 Insulator housing/
cor.par trr.ent
8 Roof enclosure
8 Control house
Control house
0 Heaters
0 Ventilation
0 Motor control centers
0 Distribution
par.elboards
' Lighting panelboards
• Starters
Screw conveyors
Rotary feeder valves
Zero speed detectors
Speed reducers
Trough type hoppers
Inner doors - drag bottom
level
Air vibrators-Navco 3 in.
Air vibrator controls
Water spray piping
Pillow block asserr\bly
Check
Initial
Data
Recheck
Rerr.arks
B-18
-------
r) Automatic back draft
dampers
s) Filter boxes - filters
t) Butterfly dampers
Check
Initial
Date
Recheck
Remarks
B-19
-------
APPENDIX B-3
ELECTROSTATIC PRECIPITATOR INSPECTION,
MAINTENANCE, AND TROUBLESHOOTING PROCEDURES
B.3.1 TR.nNSFOR.XER-RECTIFIER SETS AND ASSOCIATED EQUIPMENT
AND CONTROLS
Check the liquid level in the transforitier weekly. If it is
low, fill the tank to the level indicated on the gauge with the
dielectric liquid specified on the nameplate. Dielectric fluid
sr.ruld be handled with extreme caution.
Clean high-tension insulators, bushings, and terminals
during each outage to minimize surface leakage. Glazed porcelain
is rest cleaned with a damp cloth and a nonabrasive cleaner.
Once each year or more often, clean the contacts of relays
and dress them with a fine grade of crocus cloth.
Check the dustcp filter weekly. The air filter assembly,
easily attached and convenient for servicing, is mounted on the
control cabinet.
7r an_s_f c_r~_e_r_ Enc losure
Inspect all bushings and insulators. Replace those that are
caraged; clean those that are dusty with a nonabrasive cleaner.
Clean all interlocks and lubricate with powdered graphite to
e-.sure smooth and proper action.
Lubricate all bearing points on the ground-operator lever,
connecting rods, and bevel gears.
B-20
-------
Check all electrical connections to ensure that they are
corrosion-free and tight. Loose electrical connections can cause
electrical erosion of connections and failure of metering cir-
cuits and electrical components in both the control cabinet and
transformer.
Pipe and Guard
Remove all internal rust and/or scaling. Rust appearing on
the internal walls of the guard could peel off and fall against
the pipe, causing a ground on the secondary of the transformer.
Check the condition of the wall and post insulators for
signs of electrical tracking (arcing), dust buildup, and cracked
insulators. Clean or replace parts as required.
Check the pipe to ensure that all connections to wall
bushings and post insulators are tight and that the pipe elbows
used to redirect the pipe at various turns in the guard are tight
and secure.
Ensure against water leakage by checking and maintaining the
seal on the inspection plates of the pipe and guard.
When replacing the inspection covers, be certain to rein-
stall the ground jumper between the guard and cover plate; this
ensures that any static charge or high-voltage leak goes to
ground.
B.3.2 VIBRATORS
Inspect each vibrator for proper gas setting.
Inspect boot seal for holes or tears and replace if nec-
essary.
B-21
-------
Inspect the service sheet gasket between the guide plate and
the mounting nipple for signs of deterioration and replace if
necessary.
If boot seal or service sheet casket has deteriorated,
dismantle the vibrator-rapper rod assembly and inspect the
vibrator rod nipple for dust accumulation. Packed dust in this
area will dampen the vibrations to the discharge wires and cause
excessive dust accumulation, close electrical clearances, and
reduced precipitator performance. Check the area where the
vibrator rapper rod passes through the packing ring retainer
plate for dust or for sign of inleakage of air and/or water.
This condition is indicative of a locse retainer plate providing
an inadequate seal between the packing and the vibrator-rapper
rod or of failure of the package rings. A loose retainer plate
should be tightened and in case of gas leakage, the packing
should be replaced.
B.3.3 PLATE RAPPERS
Check the rapper assemblies periodically for any possible
binding of the plunger or misalignment of assembly. The maxim.um
amount of energy can be transmitted from ceil to plunger only
when the plunger is properly located with respect to the coil.
Any deviation will decrease the energy transmitted. Adjusting
bolts allows changes of the distance between the lower casing and
the mounting and thereby allows variation of the plunger insertion
in the coil.
B-22
-------
If the boot seal or service sheet gasket has deteriorated,
dismantle the vibrator-rapper assembly and inspect the rapper rod
sleeve for dust accumulation. Packed dust in this area will
dampen the shock wave to the collecting plate and cause excessive
dust accumulation on the plates (wires). [A boot seal is the
rubber seal that is stretch-fitted over the end of the rapper
rod. On negative-pressure installations, the boot seal prevents
air and water from entering the precipitator chamber through the
rapper rod guide sleeve. On positive-pressure installations, the
boot seal prevents precipitator gases from flowing up the rapper
and guide sleeve and entering the rapper coil tube.]
Inspect striking end of plunger to ensure that the end has
not been flared or otherwise deformed due to excessive height in
its lift and/or misalignment.
When reassembling the rapper assembly after maintenance has
been performed, make certain that the coil and coil cover are
plumb and level, and that the plunger is properly aligned in a
vertical plane on the rapper rod.
The maintenance checks outlined above apply also to wire
rappers.
B.3.4 UPPER PRECIPITATOR
Top Housing
Inspect the fan to ensure that it is working and that the
filters are in good condition.
Inspect vent elbows for accumulation of foreign matter,
which would reduce or cut off the air flow.
B-23
-------
Check access doors, inspecting the gaskets for signs of
deterioration and leaks. Replace defective gaskets and lubricate
door lugs and hinges as required.
Check that interlocks are clean, and lubricated with powder-
ed graphite.
Inspect the upper rapper-rod vibrator on the high-tension
frame to ensure that it is centered in its guide nipple and that
no dust has packed between the nipple and the rapper-vibrator
rod. If the rapper-vibrator rod needs to be centered in the
nipple, cover the insulator with an asbestos blanket, and with a
torch cut the nipple loose from the cold roof. Reposition the
nipple, centering the rod, and reweld the nipple to the cold
roof. Care must be taken that the new weld is a complete seal;'
water and ambient air could flow through pinholes and contaminate
the insulators.
Note: Whenever, it is necessary to do any welding on the
high-tension wire supporting frame, the electrical bus connection
to the high-tension support b-shing shoul d be d i s connected.. A
heavy, temporary ground, sufficient to carry total welding cur-
rent, should be solidly connected to the high-tension frame. The
disconnected bushing should be securely grounded at both ends,
i.e., in the rectifier ground switrh enclosure and at the support
bushing end.
Insulator Compartments
Energize high-tension frame vibrators and check for smooth
operation. Check field wiring and vibrator control cabinet if an
B-24
-------
inoperative vibrator is found. Vibrator insulator nuts and all
pipe plugs should be secure.
Check all nipples and seals.
Inspect all dampers in the duct connections to the compart-
ments to ensure that they are in the open position. Operate
pressurizing fan and check that air is flowing uniformly into
each insulator compartment.
The vent elbow should be equipped with a pipe plug unless
the installation is operating under negative pressure. If the
installation is under negative pressure, there should be no plug.
Inspect the elbow for dust and/or other foreign material.
Inspect the pipe and guard through the inspection hatch to
ensure that the inside surface is free from dust accumulation
and/or rust and scale. Remove all dust accumulations and/or rust
and scale buildups to prevent high-voltage arcing from the pipe
to the guard. Inspect insulators to ensure that they are free
from cracks, chips, and dust accumulations. Replace any cracked
or chipped insulators and clean dirty insulators with a nona-
brasive cleaner.
Inspect the gasket on the inspection door for deterioration
and leaks; replace worn or leaky gaskets. Make sure that all
bolts are in place and securely fastened. Determine that inter-
lock is operable and well-lubricated with powdered graphite.
Inspect upper vibrator-rapper rod - see Section 3.1.3.
Inspect the vibrator-rapper rod insulator for dust accumu-
lation, chips, cracks, and electrical tracking. Electrical
B-25
-------
tracking that has not damaged the glazed surface of the insulator
and dust accumulations should be cleaned off with a nonabrasive
cleaner. Replace cracked, chipped, or glaze-damaged insulators.
Inspect the area between the vibrator-rapper rod and the
hanger pipe for packed dust accumulations. Remove any accumula-
tion as it tends to dampen the vibration transmitted to the upper
high-tension frame. Check to see that the vibrator-rapper rod is
centered in the support pipe. If the support pipe is off center,
chances are that the weld between the lower vibratcr-rapper rod
and the upper high-tension frame has broken. Recenter the rod
and reveId it to the high-tension frame. As with the upper
vibrator-rapper rod, inspect the insulator clamp, ensuring that
all bolts are in place and tight.
Check the high-tension frame support pipe. Inspect the
round nut screwed onto the support pipe to prevent pipe r.cver.er.t.
Remove the cover plates and inspect the inside and outside
surfaces of the support insulator for dust accumulations, elec-
trical tracking, cracks, and chips. Dust accumulations and
electrical tracking that have not damaged the glazed surface of
the insulator should be cleaned with a nonabrasive cleaner.
Plate Hanger Anvil Beam
Inspect the anvil beam hanger rod clips to ensure that they
are straight. Excessively heavy plate vibrating-rapping can in
time cause these clips to bend, causing the plate bank to shift
out of alignment. This shift results in electrical clearances
out of tolerance and reduced precipitator performance.
B-26
-------
Inspect the hanger rods to ensure that none are broken,
missing, or bent. Broken, missing, or bent hanger rods usually
cause out-of-tolerance electrical clearance and reduced precipi-
tator performance. Replace any defective hanger rods.
Inspect the area behind the plate hanger anvil beam for
packed dust. Remove dust, since it can force the beam out of
plumb.
Inspect the weld between the vibrator-rapper rod and the
anvil beam. If this weld is broken or cracked, it should be
replaced.
Upper High-Tension Frame
Check bolts and welds on the high-tension frame.
Replace broken, bent, or missing support rods.
Check wire support angles for broken welds where they
attach to the spacer beam. Repair broken welds, making sure that
the wire support angles are parallel and on 9-inch centers (as-
suming 9 inch plate-to-plate spacing).
Check to determine whether the high-tension frame is level
both perpendicular and parallel to the gas flow. If the frame is
not level in the direction of gas flow, adjust at the appropriate
high-tension frame support rods. If the frame is not level
perpendicular to the gas flow, adjust at the appropriate high-
tension frame hanger pipes.
Check for excessive accumulation of dust on this frame.
Accumulations are excessive if they interfere with specified
clearances of 4-1/2 inches + 1/4 inch between the discharge wires
B-27
-------
and collecting plates or if they create a clearance of less than
4-1/2 inches between the high-tension frame and any other grounded
surface (assuming 9 inch plate-to-plate spacing).
B.3.5 DISCHARGE WIRES
Whenever possible, determine the condition of the discharge
wires with regard to dust buildup. The amount of buildup will
indicate whether the high-tension vibrators are operating at the
proper intensity.
The discharge wires should be kept as clean as is practical.
Inadequate vibrating-rapping of the discharge wires can
result in heavy dust buildup, with localization of the corona
current and excessive sparking.
A deposit on the discharge wires results from many things,
including poor gas distribution and characteristics of the dust.
Doughnut-shaped deposits often are formed. They are composed
generally of finer dust particles. Deposits on the discharge
wires do net necessarily result in poor performance, although
depending on resistivity, power supply ranee, and uniformity of
the deposit, they can reduce efficiency.
The discharge wires should be perfectly centered between the
plates from top to bottom for optimum precipitator operation.
Any broken discharge wires should be removed and if time permits,
replaced with new wires. Since a cast iron weight is connected
to each wire at its lower end, a resistance will be felt when
pulling on the wire. A wire that gives no resistance is broken.
Broken wires can sometimes be seen from catwalks located
between the collecting plate banks. With a flashlight, look down
B-28
-------
each duct noting any bottle weight that is hanging on its bottle
guide and any wires that are out of alignment.
The location of a broken wire that is removed but not
replaced should be recorded on a permanent log sheet. This
recording will save time during future outages when time permits
the installation of a new wire. A record of broken wire loca-
tions is also helpful in determining the cause of wire breakage,
i.e., if a number of wires break in the same area of the precipi-
tator, there are alignment problems. If the wire breakage is
random, the breakage is probably caused by dust buildups on wires
or plates.
The damaged wire may be cut away and the replacement wire
brought into the precipitator through the top upper high-tension
frame area, placed in the proper duct, lowered into place, and
attached.
B.3.6 COLLECTING PLATES
Whenever the precipitator is out of service and internal
inspections are possible, the collecting plates should be checked
for proper alignment and spacing. Check all hangers. Make sure
that spacers at the bottom of the plates do not bind plates to
prevent proper rapping. Check the lower portion of all plates
and the portion of plates adjacent to any door openings for signs
of corrosion. If corrosion is present, it usually indicates air
inleakage through hoppers or around doors.
Observe the dust deposits on the collecting plates before
starting any cleaning of the precipitator. The normal thickness
B-29
-------
of collected dust is about 1/2 inch with occasional buildups of
1/4 inch. If the buildup exceeds this amount, the intensity of
the plate vibratir.g-rapping should be increased. If the col-
lection plates are almost metal clean, this may be an indication
cf high gas velocity, extremely coarse dust, too high a vibra-
tir.g-rapping intensity, or too low an operating voltage for good
precipitation. This condition may be noted if a section has been
shorted out prior to the inspection.
The plate may be in effect removed from service by removing
the discharge wires surrounding it. When bellying or bowing of
tr.e plates is noted, the concave side of the plate may be heat-
treated with a torch, depending upon the severity cf the de-
formity.
B.3.7 LOWER PRECIPITATOR STEADYING FRAME
Inspect the steadying bars for cracked or broken welds where
they mount to the steadying bar support. Perform any needed
repairs.
.Make sure that the lower steadying frame is level both in
the direction of gas flow and perpendicular to gas flew. If the
frare is net level, readjust the support wires, adjusting both
until the frame is level. Place equal tension on each of the
support wires connected to adjusting bolts, since slack wires
will cause excessive sparking.
Inspect the steadying frame for downward bow in the stead-
ying bars (usually occurs after operating the precipitator at
over-design temperatures). Downward bows can usually be removed
B-30
-------
by cutting a wedge-shape slot in the vertical member of the
steadying bar angle, pushing with jacks or pulling with a block
and tackle until the frame is straight, then welding an addi-
tional piece of angle iron inside the steadying bar angle and
across the wedge slot.
Inspect the steadying frame for twisting. A twisted frame
causes excessive weight on some wires and slackness in others.
To straighten a twisted frame, grasp one end of the frame and
twist the frame until that end is straight and level. While
holding the frame in this position, weld the frame to the hopper
walls. Repeat for the other end of the frame. Once the frame
has been welded to the hopper walls and is straight and level,
using a torch, stress-relieve the frame by heating each con-
nection between the steadying bar supports and the steadying bars
until it glows to a cherry red. After all joints have been
relieved, allow the frame to cool, then cut it free of the hopper
walls. If the frame is still twisted, repeat the procedure. If
after the second heating the frame is still twisted, a new frame
will have to be installed.
When checking the lower steadying frame anti-sway insula-
tors, check the surface for dust accumulation, glaze damage
caused by electrical tracking, cracks, and chips. Insulators
with dust accumulation and/or electrical tracking that has not
damaged the glazed surfaces may be cleaned with a nonabrasive
cleaner. Cracked, chipped, broken, or glaze-damaged insulators
may be replaced.
B-31
-------
B.3.8 DUST COLLECTION POINT
In electrostatic precipitators servicing recovery furnaces,
the precipitator bottom can be one of two designs -- dry and wet.
The dry drag bottom can be constructed as a flat bottom col-
lecting chamber underneath the precipitator plates. From there
the collected material is removed by means of drag scraper and
screw conveyor and finally discharged into a rotary valve. The
use of trough hoppers with screw conveyors and rotary discharge
valves is another method of collecting dust from dry bottom
electrostatic precipitatcrs. The wet bottom construction is
associated with the conventional recovery process.
Try 2ra_g Bottoms
It is extremely important that all bearings be lube-purged
after every precipitator washdown. In the worm gear speed re-
ducer, the oil case should be thoroughly flushed with a light
flushing oil before refilling. An oil change every two to three
months is recommended thereafter. Check the oil level in both
the high speed and low speed chambers before operating. Check
periodically, with redjcer at rest, to deter~ir.e whether the oil
is at the proper level. The slow speed shaft bearings should be
lubricated with a lithium, based grease. The motor bearings are
prelubed prior to shipment. However, where the motor is used
constantly in a dirty or wet environment, it is advisable to add
one quarter ounce of lithium based bearing grease per bearing
every three months. Where it is necessary to add grease, stop
the motor. The Rex bearings should be examined and relubricated
B-32
-------
at least every six months. The Roller chain should be lubricated
manually with a brush or spout type oil can maintaining a clean
oil film at all load carrying points where relative motion occurs
to assure maximum operating efficiency. These points are between
the pin and bushing, bushing and roller, and the roller and the
sprocket teeth. Oil should be directed to the inside of the
lower span of chain in the spaces between the side bars. The
motor coupling is prelubed, but periodic inspections should be
made to assure that the coupling contains lubrication.
Wet Bottom
During periodic precipitator shutdowns, all of the liquor
should be drained into the wet bottom pan. In this way all
accumulations of saltcake present on the floor of the wet bottom
can be removed. Periodic check should be made of the structural
soundness and tightness of the acid proof lining on the vertical
sides of the pan. Check the oil level in the gear reducer of the
agitator drive. The bearings should be greased and checked. The
wet bottom liquor level control consists of an external float
chamber with a float actuated level controller. Occasionally,
soap will accumulate in the float chamber which should be cleaned
out at frequent intervals. To prevent the black liquor from
congealing in the float chamber, it is important that a small
supply of hot liquor be continually fed to the float tank.
Hoppers
The dust collecting system in bark and combination bark/
fossil fuel fired boilers consists of pyramidal and trough hop-
-------
pers. It is extremely important to establish a regular schedule
of hopper emptying at the start of operation and adhere to it as
closely as possible, preferably once a shift. If the hoppers are
allowed to fill over a 24-hour period or longer, the electrical
components may short out and precipitation will cease. Also, if
a fly ash hopper is allowed to stand for more than 24 hours, the
dust tends to pack, cool off, and absorb some moisture from the
cases. The principal problems with hoppers are 1) corrosion and
2) difficulty with free-flowing of the dust from the hcppers.
Both problems may be partially overcome by proper steam tracing
and heat insulation of the hopper exterior, subsequent care that
a constant steam supply to the hopper coils is being maintained,
and that damaged heat insulation is properly repaired. Any
abnormal buildups should be removed. If this condition becomes
chronic, it is an indication of low operating temperatures,
insufficient heat insulation, or inadequate hopper emptying.
Also it should be noted that because of the carbonaceous nature
cf tr.is dust, excessive accumulation in pyramidal hoppers can
become a fire hazard.
Screw Ccnveypr System
In recovery furnace applications saltcake is normally
removed by means of screw conveyors. In the bark and combination
bark fossil fuel-fired boilers, dust is removed by a vacuum,
conveying system. The dust is aspirated out, mixed with water to
form a slurry, and then transported away. In the screw conveyor
system it is important to establish routine periodic inspections
B-34
-------
of the entire conveyor to insure continuous maximum operating
performance. Important items to check are intake and discharge
points, flight thickness at the outer edge, and condition of
bearings.
B.3.9 PRECIPITATOR SHELL
The flue gases, emanating from the kraft pulp mill process,
contain certain acidic constituents which are extremely corrosive
to steel. Temperature control is of prime importance in keeping
corrosion at a minimum. Corrosion can become quite extensive if
interior surfaces become cool for any reason. It is therefore
recommended that thorough internal inspections be made during the
first year of operation. If interior corrosion is noted, some
means of correction should be applied as soon as possible. Heat
insulation applied to exteriors of the corroded components will
normally correct this condition.
In kraft pulp mills, process operations can fluctuate
widely. Covering the interior surfaces of side frames, end
frames, and roof with gunite will prevent damage to the steel.
The corrosive effects can be minimized by installing heat jackets
which can be heated by dry air or electric heat.
B.3.10 MAINTENANCE SCHEDULE AND TROUBLESHOOTING
Annual Inspection/Maintenance
Prior to any inspection, it is of utmost importance that the
precipitator is de-energized and grounded and the necessary
precautions are taken to ensure that the equipment control be
energized during the internal inspection.
B-35
-------
Dust Accumulations
Observe the dust accumulations on both plates and wires.
The discharge wires should have only a slight coating of dust
with no corona tufts (doughnut-shaped dust accumulations).
Thickness of dust buildup on plates is normally between 1/8 and
1/4 inch. If the plates have r.ore than 1/4 inch of dust, the
vibrator-rappers are not cleaning properly.
Pi scharge Wires_
Replace any broken discharge wires, necked-down wires, or
fatigued wires to avoid the possibility cf breaking during opera-
tion. Breakage cf ;ust one wire may render an entire precipita-
tcr section inoperative. Record the exact location of all wire
failures as well as the location of breakage on the wire.
AjLigj"..~er.t cf_ Plates and__WjLr_e_s
The plate-to-wire clearance at both top and bottom of
plates should not be less than 4-1/2 inches, while the rir.imum
acceptable plate-to-wire clearance at the vertical midpoint cf
the plates is 4 inches (assuring 9-inch duct spacing). Close
electrical clearances create excessive sparking and prevent
cptiru.T. operation.
High-Ten_s icn and Plate_ _V_i_b_ra_tor_s-Rappers
Check all high-tension and plate vibrators-rappers for
misalignment and/or binding of the vibrator-rapper rods through
the roof sleeves. Binding in this area prevents transmission of
rapper energy to the collecting plates and high-tension discharge
wires and results in excessive dust accumulations.
B-36
-------
High-Tension Frame Support Bushing
The internal and external surfaces of the high-tension frame
support bushing must be maintained free of dust to guard against
high-voltage electrode tracking across insulator surfaces. This
condition will lead to thermal fracturing of the bushings through
heat concentration. Clean all high-voltage insulators and check
thoroughly for sign of cracks; replace where necessary. All
electrical connections should be secure.
High-Voltage Electrical Control Cabinet
Clean all components of dust accumulation and lubricate
where necessary. Replace the ventilating fan filter.
Transformer-Rectifier Sets
Check the oil level in the high-voltage transformer and add
the proper oil if necessary. Check all bushings, terminals, and
insulators for dust buildup and evidence of electrical tracking.
Check the surge arrester gap setting on the high-voltage trans-
former and readjust if necessary. Interlocks must also be
checked.
Dry Drag Bottom
The pillow block shaft bearing is a double row roller
bearing pre-lubed with sufficient lubrication for approximately
one year of normal service.
Hoppers
Hoppers are present on electrostatic precipitators servicing
bark and combination bark fossil fuel fired boilers. Items to
check are dust buildup in the upper corners of hoppers and debris
B-37
-------
such as fallen wires and weights in the hopper bottom and valves.
Inspect anti-sway insulators to see that they are clean and not
cracked. If a discharge electrode weight has dropped 3 inches,
it indicates a broken wire.
Screw Conveyor
Since the highest torque is transmitted at the drive shaft
and conveyer connection, it is recommended that coupling bolts be
removed periodically to inspect for widening of bolt holes and
ber.t cr worn belts.
2ai ly_ !_.-: spect i_on_ ajnd_ Hea_dings
Record all control set electrical readings once per shift.
Any abnormalities in shift-to-shift readings may well be the
first clue of a malfunction within the precipitator. In addi-
tion, the daily log should include process operating data, flue
gas analysis, verification of transmissometer calibration, and a
record of all transm.issoneter readings.
Vibrat_cr s 'Rappers
Ensure that all collecting plate and discharge wire (high-
tension ) vibrators-rappers are functioning properly and operating
at the proper intensity level. Lack of vibrating/rapping will
result in dust buildup on both the plates and wires, which re-
duces electrical clearances and necessitates operation of the
equipment at reduced power levels. Over vibrating,'ever rapping
of the internals leads to reentrainnent of collected dust; there-
fore, it is important that proper intensity values be used for
optimum precipitatcr performance.
B-38
-------
Wet Bottom (Conventional)
Check the temperature of the piping between the wet bottom
and the float chamber to detect liquor congealment in the piping.
Hoppers
Thoroughly check all hoppers, particularly the unloading
mechanism for proper operation. Overfilling of hoppers can lead
to very serious damage of internal components. Check thoroughly
for air inleakage at the hoppers. The siphoning of cold ambient
air into the hoppers usually results in formation of condensation
and agglomeration of dust, resulting in plugging of the hopper.
A troubleshooting chart for an electrostatic precipitator is
presented in Table B.3-1.
Frequency of failure of various precipitator components and
repair times for a typical industrial precipitator are presented
in Table B.3-2.
B-39
-------
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B-43
-------
REFERENCES
1. Henderson, J.S., "Frecipitator Survey on Non-Contact Recovery
Boilers, TAPPI Vol. 58 No. 5, May 1975.
2. Industrial Air Pollution Control, Chapter 7, FEDCo Environ-
rer.tal, Inc., prepared for U.S. Environmental Research In-
fcrration Center, EPA 625/6-7S-004, June 1978.
B-44
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TECHNICAL REPORT DATA
(Please read Inuruciions on the rcvirsc bcjore completing)
1. REPORT NO.
EPA-600/2-78-210
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Operation and Maintenance of Particulate Control
Devices in Kraft Pulp Mill and Crushed Stone
Industries
5 REPORT DATE
October 1978
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
8. PERFORMING ORGANIZATION RLPORT NO.
M. F. SzaboandR.W. Gerstle
9. PERFORMING ORGANIZATION NAME AND ADDRESS
PEDCo. Environmental Specialists, Inc.
11499 Chester Road
Cincinnati, Ohio 45246
10. PROGRAM ELEMENT NO.
1AB012; ROAP 21ADL-037
11. CONTRACT/GRANT NO.
68-02-2105
12 SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final; 6/77 - 7/78
14. SPONSORING AGENCY CODE
EPA/600/13
15. SUPPLEMENTARY NOTES ffiRL-RTP project officer is Dennis C. Drehmel, Mail Drop 61,
919/541-2925.
i6. ABSTRACT The repor(. addresses the control of fine particulate emissions from selec-
ted kraft pulp mill and stone crushing facilities. The principal devices considered
are electrostatic precipitators, wet scrubbers, and fabric filters. Guidelines are
provided for industrial personnel responsible for selecting an appropriate control
device. Information on the operation and expected performance of conventional air
pollution control devices Is based on current design practice, theoretical design
models, reported performance, cost predictions, and published information.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI field/Group
Air Pollution
Dust
Sulfate Pulping
Crushed Stone
Electrostatic Precip-
itators
Scrubbers
Filtration
Fabrics
Air Pollution Control
Stationary Sources
Particulate
Kraft Pulp Mills
Wet Scrubbers '
Fabric Filters
13B
11G 07 D
13H,07A 11E
13C
131
18. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
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